Had an unfortunate experience after upgrading AVG 8.5 or 9.0 to the new AVG 2011 Free suite.

Saw the new Anti-Rootkit app included in a dropdown list. Too curious...
Ran the Anti-Root toolkit (before doing an initial Whole Computer scan). Found about 8 Rootkits. Deleted the ones that couldn't be quarantined / healed. My XP machine went into auto reboot mode. Couldn't even load Safemode into the Windows Safemode level - just kept on rebooting continuously.

Installed a second Win XP configuration - to retrieve the Nbtlog file from the original configuration.
The logfile implicated repeat Boot failure after executing the AVGIDSEH.SYS system file (Upper Case)

Tried using both the non destructive Recovery options  from Window XP install CD, Both Failed.
Should've had the Recovery CD supposed to be created for the purpose Eh?

Then used the new Paragon Drive Restore / Backup 10 to restore my entire C: drive from an earlier backup produced under Paragon Drive Backup 9.0 Express - off an external USB drive.
It actually worked ! Took about 11 hours... PC slogging it thru the night.

Thought that my original config would now boot (with complete drivers from known working config)
Still failed. Had to reinstall Windows XP and rebuild all major Security Patches & Updates to get my PC working again. Installed an older AVG 8.5 meantime - Wary about AVG 2011 for now.

Luckily all my Apps came up after sorting the CPU hogging process "fixccs.exe" problem - Google it!

Lost about 3 weeks worth of data since my last Full backup.
BEWARE PEOPLE - detected Rootkits can be both Legit eg some system files and hidden-Non-Legit.
Rootkits are DETECTABLE by AVG when INACTIVE (like a virus) but once ACTIVE are NOT DETECTABLE!

AVG need to warn people about this risk of deleting ANY remaining Rootkits after starting up the Anti-Root app Hope this helps the wider community - Very, very time consuming. Good luck.