I think the first problem that you need to address is why you can't access "My Computer". Having said that, people rely on GUIs far too much. Just open a command prompt and do a "dir" of each of your drives (C:, D:, etc.) which should show you, amongst other information how much free space you have on each disk. If you don't have much free space on your drive(s) then you need to find out why (if it's not obvious). That's fairly basic stuff.
But as I say, find out why you can't access "My Computer". Do you have the icon but can't open it, is the icon not there, or what is the problem?