I finally found the reason as to why I haven't been able to install many applications and therefore customise my HTC Snap S523. Briefly, many of these applications need to access restricted parts of the phone and of course can't be installed because these programs don't have the correct permissions. What's happening is explained here:

http://www.mobilejaw.com/articles/2009/08/understanding-application-lock-on-windows-mobile-standard-devices/

How to fix that problem is explained here. Please Note: When running the fix, you need to make sure you are using the correct .Cab file There is one for Touchscreen devices and one for Non-Touchscreen devices such as my Snap. If you look at the Update Notice at the bottom of the article, you'll see the links for the two different .Cab files which I've also linked directly to below:

http://www.mobilejaw.com/articles/2009/09/removing-application-lock-on-windows-mobile-standard-devices/

So now, after running this .Cab file I have now been able to solve one of the biggest bugbears I have with the HTC Snap; Auto Locking the phone without the need for entering a PIN number when I need to unlock it.

Here's the direct download to the .Cab file for the HTC Snap and Non-Touchscreen WinMo phones file:

http://www.mobilejaw.com/content/2009/09/MobileJaw-ClearSecurity-MobiControl.cab

and for Touchscreen devices:

http://www.mobilejaw.com/content/2009/09/MobileJaw-ClearSecurity-MobiControl-TouchScreen.cab