i know how to do this one!! ME ME ME ME! Pick MEEEEEEEE! You need to defrag it! If you have enver done it befroe, it could take two hours so leave it on over night..
To get to it, Click:
Start > Programmes (All Programmes) > Accessories >
System Tools > Disk Defragment.
What this does is orginizes all of your folders and stuff so the computer finds it easier to find them, and therefore easier to open. It shows u a diagram aswel. Then over the corse of the check, it will move all your folders (not actaully move them as of what you can see but that the computer can see). You should do it once a week aswel. That way your computer will stay runing nice and fast.

My Computer used to take 5 minutes to sign on (litterally) but now it takes 30 seconds from signing in.