Bah, thats the first thing everyone around here says!
*puts work hat on*
There could be a number of reasons for this, obviously the first is the anti-virus/malware checks. I use these, ususally does the trick:
Adaware
SpyBot Search and Destroy
Trojan Hunter (my favourite)
There is another, I cant remember, I'll check when I get to work tomorrow. Obviously make sure they are bang upto date and do a full scan, if it gives you the option of a scan after reboot, do that too.
Next, have made any changes to settings lately, doesnt matter how trivial they are, could be the smallest thing is conflicting somewhere.
Next, have you installed or removed anything recently? Sounds to me like the computer is having a problem ending a particular process, this is common if you have uninstalled something which the process is looking for, as it obviously cant find it.
Finally, have you made any hardware changes, plugged anything in, or unplugged anything, maybe a webcam/mouse/keyboard, or whatever?
Let me the answers to that lot then I'll have a go at sorting out a solution for ya!