This usually happens after a massive hardware change, and you are unable to uninstall or even install over itself to fix the problem.
I do not take any responsibilities for any damage done to your computer in following this guide.
You will need to download an application called CCleaner and install it onto your computer.
1. Restart your computer into Safe Mode (Pressing F8 during the computer boot-up usually before the Windows XP loading screen appears)
2. Go to the location where you installed AVG Antivirus (Usually C:\Program Files\Grisoft\AVG Free)
3. Select all the icons, and and delete them. If you have some Access Denied errors, take note of the name of the file that wonâ€™t delete and leave them till last)
4. After you have removed all the files, delete the Grisoft folder (Usually located C:\Program Files\) and restart your computer back into normal mode.
5. Now run CCleaner, and click Issues, click Scan for Issues. Depending on your computer it may take a couple of minutes. Click Fix Selected Issues. Do this until it detects nothing wrong.
Then restart your computer again and reinstall AVG Antivirus!