Speaking of incredibly old computers, I've always wanted to try ME.
Why would you do that.
But seriously, i dont want to run it unless im sure it wont mess any of my stuff up...
What does it do
Well, first it looks at all the stuff you could delete then it formats your hard drive.
Just kidding. It looks at all of the automatically generated Windows files, temporary files, thumbnails, deleted files that you didn't empty the recycle bin, Windows back-ups and programs you don't use any more and then tells you how much space you could free up by deleting them. You can then choose what stuff you want to delete (along with a description of what each thing is) and then windows gets rid of it.
At least that's what it does on Vista.