BleepingComputer.com: Every 3 weeks to the day - computer creeps

Hi,

I have a compaq presario desktop running vista home edition 32 bit.
I have owned this computer for about one year and a half and I have had nothing but issues.
Finally I have documented my troubles (started November 2009), and every three weeks to the day my computer just slows down to a crawl. Then after a day or two, everything is back to normal. In the process (during the slow time), i will reboot a few times (takes 10 - 15 minutes to start up at times), I will boot up in safe mode (thats quick), and then go back to normal mode and it comes up faster, but still creeps. I am in day 1 of my three week cycle. For the first time, last night, once it started creeping, and I did my first shutdown, a black screen appeared quikly, and I noticed something about task scheduler ( I could not read the details because the screen was up and away in seconds), so I am thinking something could be going on in task scheduler, and perhaps every three weeks something runs....but when I look at what is running I just see normal stuff. (system sound service, system task, TMM, and user task).
I really do not know what the heck is going on. I have spent countless hour researching forums and googling issues, but to no avail. As I mentioned, usually within 24 - 48 hours everything is back to normal. But during this time every third week, I am beside myself.
ANY suggestions are appreciated.

Sounds to me like you maybe infected with a piece of malware designed to phone home.

I have run about 5 different virus programs and nothing found.

I would suggest using the Microsoft Malicious Software Removal tool which you can get here. Or you can try a repair installation of Windows.

Did you check Task Scheduler for what's there?
Start>"Start search", type in:
taskschd.msc
Press Enter.