When re-installing you MUST use the latest AVG install file. The following is the recommended method to re-install.
1. Download the latest AVG Free installation package from [free.grisoft.com]
2. Run the AVG Free install file
3. Choose the Uninstall option and follow the setup wizard, when you get to the part to remove user settings, select it.
4. Restart your computer then...
5. Now reinstall AVG using the setup file you got in step 1 and update it.
When you were performing system maintanence... you should have also checked for drive errors... I'll post that as well as the cleanup info again.
Start by cleaning up all temporary files I prefer a util called CCleaner found at [www.ccleaner.com] which does an even better job than the one Windows has and it can also look for registry issues.
Check your hard drive for file/system errors by running chkdsk. Open My Computer and right click on Local Disk C: selecting Properties, click the Tools tab and in the Error Checking area click the "Check now" button and select "Automatically fix file system errors". It will tell you that it can't test the drive right now and it will ask if you want to schedule it for the next system startup... select Yes. Restart your computer so that it can check the drive.
To also ensure your system is operating as quickly as it can... perform a Defrag.
Other protection software, if you have any and have not mentioned it, could also block AVG from creating the settings it needs. These must be disabled while installing AVG.