spyware, cookies, heat, delete cookies, download adaware to delete spyware, allow airflow to your computer. you may want do do a disc defragmenter too.
windows is what slows down, not your PC. Although peanut is right about the heat. Windows accumulates temp files and the registry gets bulked out amongst other things.
It was more of a passing comment berating the shortcomings of windows to be honest.
Run something like linux and it won't start slowing down and packing up.
The downside is it's slightly more technical and won't (yet) run
some of the software you probably want to use.
alternatively you cold backup and save all the data and stuff u want to keep and format the computer! This starts it afresh and is as good as new. U will need the original disks which came with your pc to reinstall windows. This is also an oppertune time to install linux and perhaps make it so you can run linux AND windows. It is possible. There are programs, but i dont know what they are exactly so search around.