While a reformat is probably a good idea, a full hard drive isn't likely to stop a PC opening, there may be other issues, difficult to tell from here. Anyway here are a few suggestions :-
1st clear off any unneccesary stuff, software, files, you name it. Back it up if you want delete if not necessary.
2nd run a file cleaner like iISystem Wiper click here that will get rid of a lot of other rubbish.
3rd defragment the drive.
If that doesn't work click here for an Acrobat article on reformating, make sure all the driver disks are handy before starting, as trying to find them mid format is a pain.