i would say install another internal hard drive, like an IDE one, but, as you seem to be one of lifes. "i am so clever, i worked out what the start bar is today" kind of novices. i dont recommend it, as you would have to pay for fitting which will set you back another £30.
i would recommend that you purchase an external USB Hard drive. all you need to do is intall the driver and plug the USB lead into the socket. even my gran could do it, and she dont know a hard drive from a mouse. or how to turn a computer on, or use one.
anyway, i recommend at least 120GB+, and then transfer all your documents to the external, i.e. (word docs, powerpoints, photos etc). basically things you have saved. that will free up space on your original hard drive, so you would still be able to install software etc.
NEVER move or delete system files. i once did that, and my computer would not load up windows and i made a total mess of it. (although i was 12 and knew nothing about system files)
anyway, also try going into your control panel and removing software that you don't use.
good luck
john x