My experience so far installing IE8 RTM.  (It's NOT pretty and 2+ days wasted)

A little background.
Running XP Pro SP3 on a 1.6Ghz Intel CPU, 1.3GB ram, with Avast AV, Comodo Firewall, Spybot Tea timer, and Comodo Memory Firewall.

I have been running IE7 with the following add ons:
IE7 Pro,
http://www.ie7pro.com/

BID (Bulk Image Down loader)
http://antibody-software.com/

Orbit Download manager
http://www.orbitdownloader.com/

Plus Google Tool bar. (latest version) 

Three weeks ago I installed/upgraded to IE8 RC1, the install went perfectly and all my IE7 settings were imported without a hitch.

The improvement IE8 RC1 made over IE7 was very good, everything was faster and there were no problems with the upgrade...

This morning (Friday 20/03/2009) I downloaded and installed IE8 RTM.

The first thing I noticed it took a LOT longer to install (25+minutes) than IE8 RC1 (10+ minutes)

Then at the end of the install it said it had installed successfully but had not installed the updates it had downloaded and I should log to Windows update to install them after my computer had rebooted.

I let the computer reboot. (normal boot time is 2 minutes to a usable desktop with everything loaded and running)

Five minutes later the Desktop finally appears.
Five minutes after that every thing (AV, Firewall, etc) is loaded and running (sort of)
Avast says it's resident protection has not running. Event viewer says the Avast service failed to start.

I reboot again. Again it takes 10+ minutes to boot.
Avast is now running properly.

I click on the IE icon in the task bar.
Statbar immediately shows CPU usage jump to 100%
The mouse becomes sluggish
IE8 RTM still hasn't loaded after 5+ minutes.
I press CTRL ALT Delete to get task manager running.
IEB RTM finally loads but it's a blank page, with a single tab saying connecting....

The Address bar has my homepage url in it.

3-4 minutes later my home page finally loads and Task Manager finally starts.
The Task manager running processes list show IE8 RTM is using 100% of CPU.

I try going to a few other websites on my favorites bar, they load SLOWLY in the current tab; I try to start a new tab, the tab just shows the connecting message again.I try to close the tab after 5 minutes. Nothing happens. CPU still at 100%

I go to the Task manager and cancel IE8 RTM process. CPU still at 100% and I notice that IE8 keeps showing up and disappearing as a running process.

I decide to reboot the computer again.
I go through the same scenario above several more times with the same end result.

I then decide to uninstall IE8 RTM, and roll back to IE7.

This goes OK, and once I'm up and running with IE7 everything is back to normal.

I then re-install IE8 RTM (just in case the first install hadn't worked properly.
Again I get all the problems above.

I uninstall IE8 RTM, roll back to IE7, then reinstall IE8 RC1.

The computer is now working as it should and IE8 RC1 is working perfectly.

I see in the Microsoft IE Usenet group that others are reporting the same/similar experience I have had trying to get IE8 RTM working...

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FOLLOWUP 03:05pm 22/03/2009:

After reading various forums like MS IE.general News Group, web forums,etc about the problem, it appears to be affecting Windows XP SP3 users with Spybot S&D and Spyware Blaster installed, I followed some of the possible fixes that say to remove Spybot S&D and Spyware Blaster including de-imunising the registry before uninstalling. (turn off spyware protection in the programmes before uninstalling, this removes their restricted sites list from the registry)

I also ran a registry clean to remove orphan Registry entries for both programmes.
Didn't make any difference to a fresh install of IE8 RTM

So I decided to do it the 'hard way.'

I shut down everything except Avast and Comodo firewall
Uninstalled Google Tool bar, IE7 Pro, Orbit and BID download managers, Comodo Memory firewall, IE8 RC1...

Ran a new install of IE8 RTM to upgrade IE7...
unticked the option to run Malicious Software tool and download updates at the beginning of the install.

Install took 5+ minutes (thats a lot better than the 25+ minutes it has been taking)

After install the reboot was the normal 2+ minutes (not the 10+ minutes I have been getting with previous install attempts)

Had a usable desktop immediately. Mouse isn't sluggish, all tested programmes (Windows Explorer, MS Word, Graphics programmes, Media players, etc) running as normal.
Event Viewer not showing any errors...

Started IE8 RTM. Started without any delay and loaded homepage immediately.
All my favorites from IE7 are available, all my links are available and the websites load immediately.
New Tabs open properly.
Shutdown of IE8 RTM is immediate.
CPU is not running at 100%

Started reinstalling the uninstalled programmes one at a time, starting with  Comodo Memory firewall. Installed and running OK

IE8 RTM starts and runs without any problems

Reinstalled the other programmes one at a time as well, without any problems for the programmes, XP, or IE8 RTM

I decide NOT re-install Spybot S&D, or Spyware Blaster yet.

Download and install Windows defender without any problems for programme or IE8 RTM

03:45am and two days after I downloaded IE8 RTM I finally get it to work like IE8 RC1, but without essential Spyware defence programmes installed.

So the problem still exists, but the only work around appears to involve running a compromised computer that may be at risk of spyware infection.

Hopefully Microsoft can find out why IE8 RTM has compatibility issues with some [essential] anti-spyware programmes.

It seems like they may have enabled something that was disabled in IE8 Beta versions and RC1.

Or maybe the put a bug fix in the RTM version that is causing the problems.

Maybe the restricted sites lists of Spybot S&D and Spyware Blaster 'confuse' IE8 RTM when its checking for restricted sites; Maybe It cant read/check restricted sites it hasn't installed in the list?

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FOLLOWUP 03:34pm 22/03/2009

It appears a new 'function' in IE8 RTM called LCIE is the possible cause. (as mentioned in the MS IE.general newsgroup, and several MS web forums). 

It appears that this new 'feature' [Loosely-Coupled IE (LCIE)]
doesn't like old anti-spyware programmes that add their own restricted sites list to the XP registry.

Maybe it's time for an upgrade to anti-spyware programmes to not use this method of spyware protection.

I've seen LCIE working already when incompatible websites have crashed, all that's been shut down has been the tab they were loaded in, which is a lot better than the whole browser crashing.

The mystery is why LCIE in RC1 wasn't having problems with SpyBot and Spyware Blaster, but the RTM version is.

Anyway I now have IE8 RTM working as it should.