Hi again.
Now it's starting to sound like it might be some kind of malware.
Run SuperAntiSpyware in safe mode and see what you find.
SUPERAntiSpyware.com - Downloads Have you gotten any messages about updating you antivirus lately?
A friend of mine had a Trojan about a month ago that wouldn't let him install any security stuff, including SuperAntiSpyware.
In order to run it I had to install in on a thumb drive on my computer and run it in safe mode on his.
It did find and remove it.
I suppose that the same thing left over from Nod could be blocking AVG as well.
If that's the case and you can't figure out what it is then it may be time for a repair install of Windows, it only takes about 20 minutes but I'd do it as a last resort.
When I do a repair install I do it from a new user ID, it's a pain because you have to transfer you address books, favorites etc. but it keeps Windows from picking up junk from your old one.
I create the new ID reboot, into the new ID, turn off all startup items and services, then reboot again in this configuration and then start the repair.
This gives you the cleanest possible install without deleting anything from your hard drive.
I've occasionally found some software had to be reinstalled after this but not often, non of my Adobe stuff refused to run afterwards.
After the install you will be able to boot into your old one to retrieve anything you need from it.
Once the new one is up and running you can delete the old one.
Note: I personally have never done a repair install in Windows 7, but many like this in XP.
Mike