I haven't personally heard of a virus that "eats the words as you type" but I am happy to be corrected by someone.
I wouldn't buy anti-virus either - AVG:
AVG Website is a great FREE anti-virus product. I have used it for at least 4 years and it has never let me down.
As for the cleaning up the hard drive - if you have another drive on your PC why not move all the "non-program" software such as images, music, documents etc to the other drive. This will free up quite a bit of extra space on your C drive
