Pretty much what everyone here said to do, do. I'm not a computer expert, but I've been using them for 14 years, (and please, this isn't a brag. I'm still stupid don't worry). Anyway I have a Mac and a Windows machine. When my Windows machine starts to feel lethargic, I run CCleaner, and clean everything. My temp files, my cache's, and my registry. I know that some people get nervous about letting a program clean files from the registry, but CCleaner lets you back up the registry before you make any changes, and it does help your PC to help get some of the junk cleaned out of the registry.
Also, get and run Spybot - Search and Destroy. The scan can take abit, but if you have any spyware, adbots, bugs, trojans, etc, then it will find and remove them.
And as others have said, DEFRAG! I use Defraggler: http://www.piriform.com/defraggler
It's fantastic. Also, you can find CCleaner here: http://www.piriform.com/ccleaner
From the same guys that make Defraggler and CCleaner, I also use Recuva: http://www.piriform.com/recuva
Useful when you accidentally delete a file, and want to get it back.
And hell, while your at it, this is a great tool as well: http://www.piriform.com/speccy
It lays out your computer specs, from operating system to graphics card, to RAM memory. Very useful when either buying upgrades for your PC, or just showing the specs of your machine off.