option default hard drive

I have two hard drives, (I guess this would work the same for partitions), But I want to put my Program Files on one drive, and the My Documents folder on the other. The only problem is the machine, (along with most software) automatically default to "C". There should be some way, maybe through a regedit or something (preferably something less dangerous) to set the default drive for most types of files, so that the machine will default to the drive of choice. I moved all my documents to my drive "F", but I had to leave the system file for documents on "C". I changed all paths to indicate "F", but after a couple of reboots, it went back to defaulting to C, so I had documents on two different drives. I would not think it would be hard to do this.
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