Computer randomly SLOWS!!

My computer has been having this horrible slow down problem since the second month ive had it, it has been 2 and a half years now and the problem will not go away.

Mostly everyday my computer will slow way down to the point nothing will work. clicking icons, pressing buttons, ANYTHING will just simply not work. I get the spinning blue icon that shows my computer is thinking about it and over time everything will go bright white saying "Not - Responding" This happens to all of my programs including Windows explorer itself.
This happens only at random times, usualy when the computer has been on for hours.
Ive checked task manager to see what is eating up my CPU (because the small yellow blinking light on the PC case for the CPU is on solid light and no longer flashing) But all of the tasks are at a perfetly reasonable memory usage and the CPU says it is working at 15 - 25%. The CPU light sounds like it is doing some serious thinking or as if i was running a full system scan.....but i have done nothing!
Ive checked the temperatures of my PC and nothing is over heating. I open it up anyhow and clean any dust out that i find. Ive even unmounted the fan atop my CPU and cleaned out the dust in those hard to reach areas beneath the vent.
Ive scanned numerous times with the AV which never finds a problem. The registry has been scanned and any errors fixed but still no help.
Also ran diagnostics tests and even ran memtest on my computer and it hasn't found any hardware or memory issues.
The computer will never shutdown when it is slow like this so i am forced to a cold shut-down. I disconnect the power and drain any power that may be left inside. I wait a minutes and plug in the power and boot up my computer. And somehow it KNOWS to be slow!! I wait for a long start up, long log in, and a long wait for my account and the computer will still be just as slow as before!

Leaving my computer alone for about 2 - 3 hours and it usually clears back up and is fast once again like it is suppose to be >: O
Normally the computer is quite fast, besides the random slow downs that take hours to clear up.

If there is anything else you need to know about my system then let me know and ill post my specs.

Test with a clean boot and report results.
http://support.microsoft.com/kb/929135
Make sure that the AV was a full scan and it was updated
Download and make a full updated scan with malwarebytes.

http://www.vistax64.com/tutorials/81176-speed-up-performance-vista.html

Possibly it is scheduled tasks slowing down the computer
Completely unattach from the internet to test. See if slow down continues.

The slower my computer gets, the lower usage of CPU it is using. Right now it is at 5% CPU usage and yet the light is still solid. Ive heard of some systems that purposely slow down when temperatures are to hot to prevent damage. But the computer is far from overheating as my house is quite cold at the moment.

Im going to try some of the steps you showed. A clean boot Ive done before but like I said, the computer slows at random moments and not at the start up unless the computer was slow previously before shutting it down, then somehow it remembers to be slow when turning it back on.

Ive also unattached the internet before when this has happened (no such luck =C )

If overheating is any concern at all download speedfan and check the temps.
http://www.almico.com/speedfan.php

Hi,

I think that the two links suggested by Richc are excellent starting points.
If it continues i'm sure there will be other avenues to follow.








I do not seek to follow in the footsteps
of the men of old,
I seek the things they sought‘

Basho (1644-1694) Japanese Haiku

Hehe, thats the program i am using at the moment.

I did the "Increase disc performance" Option given in the link you gave me. My computer seems to have came out of the slow spell a few minutes after doing the disc increase.
Im beginning to wonder if my computer randomly decided to work again or i really fixed the problem... If this continues to happen later this week then ill try the other steps given and if they do not work then ill come back and reuse this forum post for any further assistance.

Work at your own pace, we will be here for you.