Avast forums has posts about weird lag getting injected from their firewall. Could be something going on like that if you've recently updated your security software. Could check the support forums for that software to see if anything is being reported there.

If you've got anything overclocked or otherwise tweaked to use turbo/performance settings, try easing up on the settings, just to rule it out. May not necessarily be overclocking in the normal sense--could simply be running your memory on a turbo or high performance profile, or otherwise tweaked it to be running tight timings (CAS/RAS and such) in the BIOS. My XP box started freaking out like that recently. Ultimately had too loosen up the RFC timing 14 points to stop the random lockups, then had to also drop the FSB a little (dropped CPU from 4.0 to 3.96 GHZ) to finally get it to smooth out again.