I had been experiencing the same slowness on my CPU but couldn't find any spyware, etc. but it got slow especially after I tried installing some free crappy anti-virus program. Anyway, one thing I found that helped my system was to do turn off all the start-up programs, and that seems to have helped. Anyway, here's what I did:
(I'm Running Windows XP SP2)
Start Button > Run... > type "msconfig" > hit "OK"
Clicked the "Startup" tab across the top.
It would probably be a good idea at this point to make a list of anything that is not checked in the list.
Clicked the button "Disable All"
You'll notice you can "Enable All," thats why I'd recommend making a list of anything not checked.
At this point I got a message about not being able to access a service, unless I'm an administrator, which I am, so I hit OK. Twice. Jeez.
You'll be asked to restart.
When your system boots up again, you'll see a message that tells you the startup has been altered or something. Say "OK," (I left the box marked "Do not show this again" unchecked as a reminder of the items I removed from auto-starting in case 6 months down the line something didn't work and I don't remember what I changed).
At the next prompt, I hit "exit without restart."
That seemed to get things back to normal, and so far I haven't run into anything that doesn't work without those things running as startup items.
Can't say if this is really safe, but updates still work, virus updates, etc. So far so good.
You can always go back to the msconfig and turn it all back on if something won't respond.
Anyway, hope that helps or is of use to someone else out there.
Please don't sue me if it goes haywire