Agree with USAFRet, you really should backup those files somehow before proceeding, it's very easy to wipe the entire drive during a fresh install.
If the system has a DVD/Blu-ray combo drive you can burn the files to blank discs for safekeeping, if not check the amount of stuff you have and see if you can move it over to a USB drive.
If you really can't backup then be very careful during the install to only install to the 'C' partition, as I recall you can rename volumes through the disc properties dialogue, I suggest you rename both 'C' and 'D' to make it easier to recognise which you will install to.
Whatever you decide, don't delete any of the drives or partitions, just format the one you will install to. Windows will allocate some extra reserved space during the process for its own dark, sinister purposes and that will show up as a separate entry in the HDD listing, just install to the formatted 'C' drive and ignore that entry.