While using my computer, every once in awhile it will freeze, and then after about 15-30 seconds, unfreeze. This happens quite often (probably once every minute or so). What could be the problem?

What are the system specs, including Vista version and Bit version

It's Vista Home Premium running on an AMD Turion 64 X2 Acer Laptop. ATI Radeon Xpress 1100 with 1GB ram and 120GB hd.

Thanks Rich, I tried the ewido one and it found tons of tracking cookies right when it started. Now everything seems to be fine. Thanks to everyone else as well.

Right click the task bar, select Task Manager, then sort the process list by CPU - I'm guessing something is hammering your CPU intermittently causing said freezes. - this could be a malware infection