There are sooooooooooo many more things than just a virus that can slow your computer down. temp files and cookies can be deleted-but also look at the following-how much free space is on your hard drive? if 20 percent of it is not empty the machine will run slower. How much memory do you have? if the amount of memory is not adequate for your operating systems ( 512 GB min for windows XP, 2 GB mimium for Vista -not sure what the minimums are for Windows 7 ) and you do not have enough hard drive space vacant for the paging file or swap space to reach its maximum size your computer will run slowly. If you are using broadband or DSL and the machine is connected to the internet when ever you are booted up its likely that you have pirate processes running as well-Pirate processes are when someone access your computer to use your memory and processing speed to complete their task-not quiet the same as a virus since a virus damanges your computer-pirate processes generally just makes it slow.
Any of those things can slow your machine-and downloading a free anit virus wouldnt get rid of most of them.