Clean the gunk out of the mouse! (My grandmas solution to all computer problems)
But for real, check for viruses, I got a free registry cleaner from Eusing and I use Malwarebytes' Anti-Malware, it's free.
I've been using Avira for quite a while but
then when i started it up againthis morning, it
stopped working every time i pressed full
system scan, and the last time i had a full system scan
was last month so ya....
I had a case where my 3MB internet was always constantly locked at 100kb.
When I got a new WAN IP by leaving the modem unplugged for a few hours, I had normal internet again.
Just a possibility it might be the same for you.
Unless the issue is caused by something that's not a virus (possible), not filesystem fragmentation (I extremely doubt this is the case), and not low space (in which case he'd get warnings from windows about it).
We should use the process of elimination to FIND WHAT THE PROBLEM IS FIRST. All we have right now is a symptom.
Check and fix all those entries, then go to the computer systems section and look at this sticky, and follow the "advanced removal" instructions (after the intro and setup), then follow the "future prevention" instructions to prevent this from happening again.