You'll need to backup all the stuff you want to keep onto an external drive.
Then you format your computer, plug in external when you're done, copy data back onto your computer.
There's no other way around.
If you don't have external, but have 2 HDDs
You can put your data on a second HDD (if you have two, and there is enough space on the second one).
Then once you've formatted your C:// (main hard drive) you can plug in the second one again, copy the data across (
to C://, and format D://
how to format
(Start -> right click Computer -> Manage -> Disk Management -> Right click D:// -> Format)