Computer Goes Into Slow Motion Mode

I have been having this problem for past 2-3 days and this is very very random. At times, my laptop would just go on a really slow motion mode where even switching windows or typing a word would take seconds / minutes and it would be fine after a few minutes. This has happened several times and I have tried to keep an eye on which program could be causing it and narrowed it down to Firefox, MSN Messenger or uTorrent because they were running at every time this problem occurred. In fact, once i forcibly closed Firefox and computer was fine. But at different times of when this occurred, either or none of those problems were running so those processes could be ruled out. When i checked Task manager, 'CPU Usage' was shot up to 100% but under 'Processes' none of the programs were hogging down the computer.

I ran Ad-Aware and Spy Sweeper and none came up with any suspicious/malicious threats. Ad-Watch was kinda slowing down my computer so i thought that could have been it but this has occurred several times after Ad-Aware's uninstallation. I have also ran a virus check and nothing came up. :s The weirdest thing is that if i forcefully shut down the computer and restart.. it still starts off in a really really slow motion mode so the slow effect somehow still carries on after computer's shutdown. I ran HijackThis and ran auto-analyzer but that came up with nothing either.

I would appreciate any input or any ideas towards solutions anyone may have.. this is getting really annoying even though i have been very patient. I am running Windows XP on a HP Tablet and all my windows update/security/anti-virus are up-to-date. I haven't installed any program in past 2-3 days except Ad-Aware and Spy Sweeper and i am never prompted for any pop-ups or anything, so being online or offline doesn't make a difference, it seems. Oh and the only processes on top of 'Mem Usage' in Task Manager are usually McAfee Service and Task Manager itself (after firefox's termination).