Slowing problems can sometimes be hard to find and can be caused by several different problems. Most of the time they are caused by adware &/or malware.
When you say you run Ad-Aware and you didn't see anything that wasn't suppose to be there, if ad-aware found anything then there is a good chance it isn't suppose to be there.
Lets start with making sure you are clean.
See this article:Taking out the Trash
Show all Files & Foldershttp://www.bleepingcomputer.com/forums/ind...showtutorial=62
Try these free tools.
Trendmicro (free virus scan only)http://housecall.trendmicro.com/
Adware SE (update after installing) http://www.lavasoftusa.com/software/adaware/
Spybot S&D (update after installing)http://www.download.com/Spybot-Search-Dest...4-10122137.html
If you are running a desktop, open the case and make sure the fans and everything isn't clogged up with dust.
If Laptop make sure the fan is running on the processor and you are not setting it where air is restricted from it.