1. Install CCleaner and before running it, check/uncheck the items you want to be removed or want to keep. Run scans for Windows, Programs and Registry. The amount of data will depend on the time you haven't deleted junk files
2. Go to Start\Run\and type "%temp%" (no quotes) and hit Enter\select and delete any files you find in the user account temp folder.. exit that folder and go to the C:\windows\temp folder and delete the files you find in it.
3. Go to Control Panel\Internet Options\General tab and Delete the Browsing History... CCleaner should delete this but it sometimes doesn't delete all it should, so to make sure all the Internet Temp Files are deleted you should delete them from Internet Options. You'll need to press Delete two times but before pressing the second time, see and uncheck what you want to keep (E.G. Passwords).
4. Temporarily disable Hibernation in Control Panel\Power Options\Hibernation\Unckeck the Enable Hibernation box.
5. If your system is running without a problem, you may not need the System Restore saved points which can take up a few GB of hard disk space, so you may want to disable System Restore temporarily. Control Panel\System\System Restore\check the Disable System Restore box
6. If you still need to free more hard disk space, you may want to reduce the disk space assigned to Virtual Memory Control Panel\System\Advanced\Performance-Settings\Advanced\Change\Type in the disk space size you want to temporarily assign to the Virtual Memory. If you have more than 2GB or RAM you can probably disable Virtual Memory completely while you defrag the disk.
7. Bakup your files and folders to removable media... Data CD, DVD, Flash Drive, etc.
8. Another option is to compress your files and folders with a zip, rar application. WinZip, WinRAR, 7z, etc.
8. As last resort you can temporarily enable compression on the System Drive C:\Properties\General\put a check mark on "Compress Drive to save disk space"\click Apply & OK.
9. If you only have the System drive (C:\) you can create a new partition, and enable compression in it instead of compressing the C:\ drive, and as you move your files and folder into it they will be automatically compressed.
Drive compression should free as much as +/-7GB per every 20GB of disk space.. at least that's what I got on one of my partitions. I also compressed the system partition a couple of times and did not experienced any problems due to it.