My first guess is that the speed decrease is due to spyware. These nasty little programs often run in the background of your system and rob you of the system performance. Some can be very difficult to remove.
In addition to the Spybots, I suggest you run the Ad-Aware from Lavasoft
Ad-Aware and once that is complete, run the SpywareBlaster.
SpywareBlaster Be sure to run the updates after installation to get the latest detection files.
This site offers some instructional tips about using these programs.
Ok, Teach MeAlso, try this link to check for browser parasites:
Check for parasitesFinally, you can use the "Hijack This" utility program to ferret out really persistent spyware. Be sure to use this in conjunction with any of the Help Forum sites shown on the home page:
Hijack ThisIf, after all this you don't see an increase in speed, then I would suggest you look into the speed tips by tweaking with your bandwidth.
I hope something here helps!
Dwight