Try running Disk Cleanup, also look at Users Temp files.
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Select both boxes.
The Program Files folder of AVG should be about 31MB in size. There is another location, C:\ProgramData\Avg7, that is about 15MB in size, and one more, C:\ProgramData\Grisoft, that is about 13MB in size, but can vary slightly depending on size of definition files.
As you have been trying various scanners, remnants from past uninstalls could interfere with any new install, now or later on.
Symantec is "Nortonorious" for this type of behavior, and have a special cleanup program either on their install CD or downloadable from their web site.
Using the Norton Removal Tool
One sure way of knowing if AVG is installed and working correctly is the presence of a big green tick on the Control Center page.
This is the type of message I get from AVG when it checks E-mails in Vista -
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.476 / Virus Database: 269.10.2/894 - Release Date: 10/07/2007 5:44 PM
When I paid for Virus Scanners, I loved NOD32, but my surfing habits and general experience allow me to relax the security a little and settle for a good free scanner.