How Windows Assigns Drive Letters you discover that there are rules about that and that each install can have differing drive letters.
Now that you know that you can't count on C being C or D is D you only need to know this -> "We only lose what we don't backup."
Installing any OS is not safe enough to write "having your files on another drive or partition is safe." That's some urban legend that won't die. Yes, it works most of the time but for those that it didn't they learned it the hard way.
Try this. Unplug all drives you don't want Windows to touch. Place the target drive on lowest channel of SATA on that board and begin the install followed by all the drivers then when it is all working, power down and connect one drive, boot up and see what happens.
Hey, I am planning on moving from windows vista home premium to windows 7 professional. So i will have to do a clean install of windows 7. I was wondering, how do i know which of my local disks my operating system is on? I have a D, E, F, and C. I'm not much of a computer guy, but "Local Disk C" is the largest. (I ask this because I know when i do the custom install of 7 its going to ask me to choose the disk my current Operating System is on)
Also, what type of files should I backup? Can i do this via a 4GB USB drive? I have all the reinstallation CD's my computer originally came with. Do i need to reinstall all of those?