Maybe you need to clean the thing up? I mean, clean it from dust.
I read an article somewhere that cleaning up your computer can boost it's performance overall, at least similar to what you feel the first time you see "Welcome to Windows, let's spend some time configuring your computer"
Formatting everything might work, but if you have lotsa programs (especially those exlusive-to-one-key type, like Kaspersky or something) it's not the best option, try to defrag our hard drive first, usually it boosts speed a bit.
Harmful objects like virus and such might slowdown your computer too, try to check your taskmanager for unknown processes, usually booted by USER(you, unknowingly of course, its a virus), scanning for them is a good option too, although usually viri target the the data inside your computer, not your hard drive.
If you have too many files within your hard drive, slowing down is something you can't escape, you can't expect your computer to be as fast as the day there's 80% free space in your hard drive now that you have only say, 20% left. In this case, the best option would be go get additional RAM, or get a better processor to keep up