Under a typical installation, your desktop resides on the C: drive.
If you go to Flying Window (formerly the start button) and click on 'Computer', you should see your C drive shown in there, with a bar graph. Is that bar graph in red? If it is in red, then you are within a few percent of being full.
If so, click again on the Flying Window button and in the search bar, type in Disk Cleanup. First tell it to clean up only your user account. If that doesn't clear enough, then re-run it for your whole computer.
Hope that helps.
