problem with windows date and time

I use windows XP with SP2.Since last wek, I observed that date and time shown by system has changed to 31.12.2001, 11.14PM. I tried to correct through controm panel. But it reverted back after I switched off pc and started again. I changed CMOS battery,but to no avail. As a result, I am getting message for my free AVG antivirus "Reinstall your licence number incorrect" meaning my pc is at threat Please guide me. I am also unable to download SP3 from windows updates.

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