This isn't entirely complete, but it will get you started:
Defrag
Disk clean-up
Check Errors
Check for unwanted Processes/Start-up items
Updates (i.e. MS automatic updates)
Anti-virus Scan
Anti-Spyware Scan
Programs List--check for applications you don't use any more.
Update Drivers--especially for video, audio, and CD drives
Registry Mechanic/Error Doctor/RegFix (Trial Versions)
Computers slow down over time because they need simple maintenance, such as above. Computers don't slow down simply for being "old" (unless something flat out breaks).
Something else to think about is how much RAM you have; it may have been plenty at first, but perhaps you have added a lot of "stuff" that runs in the background all the time, thus needing more RAM.
Without looking at it, that is all I can say, as there can be 5,000 things that are making it slow. So, start with the list I gave you above, and that should do something noticable for now.

Good luck!