|Don't forget reserved space. Space used for paging file (virtual memory) and System Restore. Depending on the amount of memory installed, you may be able to reduce the paging file to the size of your RAM installed. You can also reduce the system restore reserved space from the default of 12% of your disk size to about 8% of your disk size (this will reduce the amount of available restore points by deleting the oldest ones) and you should be Ok. These can help in the short term, but the real answer is to add drive space to your system. If this is a desktop, just add a second hard drive and move all of your personal files over to the new drive, leaving Windows and your programs on the primary drive. In the future you can even install programs to the new drive and if the install medium is available, uninstall and reinstall other programs to the new drive if needed.|
If this is a laptop, your choice it to get an external hard drive and move less important files off the computer onto the external drive (and use it also as a back up for the important ones). OR you can replace the hard drive with a larger one and use the hard drive mfg's utility to clone your hard drive onto the new hard drive and switch them out (you may need a desktop to do the actual cloning though).
Depending on the age of the system and how well it currently works, you may consider replacing the system as well.
You have to be a little bit crazy to keep you from going insane.