PC Lag Problems? (Help!)

All of a sudden my computer has lag when using messaging programs like AIM and MSN. Occasionally with web browsing (firefox) is also lags. What it does is, while I am typing, is freeze for 2-3 seconds. Sometimes my type appears once my PC unfreezes other times it does not.

I tried reverting my PC to a restore point and it stopped the freezing occurrences for approximately 15 minutes and went back to freezing/lagging.

Anyone have any idea what went wrong to make it start doing this? I did no install anything and I have ran a few Spyware/PC optimizing programs(defrag hdd/registry, repair, etc…) to see if they would help but they did not.

Thanks,
Flash-matic