Does the computer boot OK? When booting, the BIOS looks for the Master Boot Record, which then looks for the Boot Sector, which then starts the boot process of Windows.
Could this just be that AVG can't read it, but it's working OK? I haven't used AVG for a while, but do recall some warnings from it that weren't very significant. It may be that AVG just isn't capable of reading your MBR and Boot Sector.
You can repair the Master Boot Record and the Boot Sector by using the Recovery Console (instructions here:
http://www.bleepingcomputer.com/tutorials/how-to-install-the-windows-xp-recovery-console/ ) by using the "fixmbr" and "fixboot" commands. But I wouldn't suggest doing it unless you're having trouble booting up.