The problem is you didn't make proper use of your drive space. What was the point of partitioning if you weren't going to take advantage of it? Drive C should be for Windows, drive D for programs & drive E for storage. There are numerous reasons why you should have done it this way.
To add to the above comments you can free up space by uninstalling your programs and re-installing them on the D partition. All you need to do is use Custom install or browse when installing. The key is to READ the screens when installing.
If your 'My Documents' folder is huge, you can move it to another drive. Tweakui (free) has the option to move system folders. If you are interested in doing that, look in the Tweakui 'My Computer' section.
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