Antivirus programs do not block time sync requests by the OS... your firewall could, but this is usually averted by the firewall allowing (someone else correct me if i'm wrong here) the svchost.exe process access to the 'net.
The CMOS battery dying is much more likely. If you have a multimeter handy, pull the CMOS battery off of the motherboard (after unplugging the system of course) and test it with the multimeter. If you're not getting a constant reading of 3V DC +- 5%, you're battery should be replaced.
If you need to replace it, just about any consumer electronics store should have them. Just ask for a "CR 2032" and they should be able to find it for you.