A couple things that eat up disc space quickly on small disks:
Hiberfile: If you do not hibernate, you can delete the hiberfile which will give you approx. 8GB space.
You can still "sleep" the PC
Pagefile: I wouldnt turn it off... but make a static smaller PF. Depending how much RAM you have, Windows sets it at 1.5x RAM installed & This isnt necc. IMO.
If for ex. you have 4GB RAM, you can try setting it to 1.5 or 2GB . smallest and largest the same whatever you choose.
this should free up approx 4GB. this should be more than enough
The above will give you back approx12GB HD space.
System restore: Do not disable, especiAlly if you have no alternate back up means, BUT, you can lower how much space it can use if space is a real issue..
If you done a upgrade, you can delete the Windows.old folder after you get what you need from it.
TBH, you better off just doing a clean install though