Start by running the cleanup tools that MS already gave you with windows.
1. Disk Cleanup
in the START > Programs > Accessories > System Tools folder.
The first time you run Disk Cleanup, check all the little boxes before
you click OK, to run the program.
2. Disk Defragmenter, found in the same folder.
Click on the C: drive, then click "Defragment" (NOT Analyze)
The will free up a lot of valuable space.
Then you should go into your Control Panel, the Add/Remove applet and DE-Install any programs that you don't any longer use or need.
That will free up even more space.
That's not the whole story, but it's a beginning.
The Shadow
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A person with experience is never at the mercy of a person with an argument.