what you can do is go to start > programs > accessories > system tools, run the disc cleanup wizard, make sure all the boxes are ticked to get rid of all the unnecessary crap. Then run the disc defragmenter. Be warned tho, depending on how full up with shite the hard drive is, these processes could take a couple of hours! Make sure its plugged into the mains, and its not set to go into standby after half an hour or anything.

Then get rid of any virus programs you have, and download the free home edition from avast.com.

That should get you well on the way!