Using Avast 4.8.1368 on a Windows 98se PC, using VPS 101010-0, system date/time is fine, everytime I reboot or turn on the PC, Avast claims the VPS is old and wants me to update. Attempting to update manually indicates I have the latest Program and VPS updates.
Try a repair of avast. Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow. This has in the past resolved this out of sync issue between reported and actual virus definitions version.