Well, Tews is right but if you are keen on upgrading then do the following to recover some more space on your hard drive.
1) Turn off hibernation. Run CMD as admin, at the command prompt type: powercfg -h off
2) Turn off system restore.
Also, see this thread Not enough space for upgrade installation
Also, does your computer have an OEM recovery partition? As a last resort, you can delete that partition and use that space but make sure you make and test recovery dvds before doing that.