Uninstall some of the programs you no longer use, transfer music files, photographs and documents to an external hard drive and use a small free program called Ccleaner to clear up temp files etc..
Look for a separate backup partition .. and move My Docs to it.
(R Click My Docs/Properties/Move ... all .. it's a seamless change)
Back off the S Restore settings a bit .. to 10 or 11%
You open Disk Management off of Administrative Tools (Control Panel)
Look at the Discs listed to see what you have.
It's possible you have a partially used back-up partition (drive) on the same hard-drive as the OS.
If you are installed on C .. it will probably be listed as D.
All the advice you have been given below is short term, you should drop it into a computer shop have them fit a bigger hard drive and clone your old hard drive so you don't loose any data.
You should probably backup any data before you hand it over, its better to be safe than sorry.
This repair shouldn't cost anymore than £100 including the hard drive, hope you get it sorted.
I wouldn't even do that, why take it to a shop when you can do it yourself? Get yourself an external hard drive and download to that the big files which you want to keep but you don't need all the time.