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Is "D" a partition or a second hard drive? If it's a partition it shouldn't make much difference so long as it is set up correctly and all program installs recognise D as your system drive. If you have an install CD then a simple format and re-install (using "C" as the system drive) will do it.

Hello and welcome to the forum.
It's unusual to have an OS installed to the D drive, it's usually on the C as Windows assigns drive letters in order as below

A: Floppy Disc
B: Second Floppy (Rarely used)
C: Main Hard Drive
D: CD/DVD drive
thereafter windows should assign drive letters as hardware is installed i.e. the next drive installed be it a Hardrive or CD drive will be E:

Could you post your system Specs to help our brilliant team of members and staff assist you further

Like Hengis said, if it is a partition, then it doesn't make any(or much) difference. If you insist on having it on C drive, then perhaps you should just change the drive letter of D to C instead of transferring files to C drive. By the way, what files are you intending to transfer to C drive? All files should be movable readily to C drive except for the system files which the OS require in order to run.

However, if it is another hard disk, then i guess you'll just have to reinstall the OS on the other drive, that is if you want it to be on C drive. I don't know much since I never had more than one hard disk on a single unit.

Thanks for the info guys, I believe the chap I bought it from had 2 pcs running and transferred over as a partition . Got no discs with it,so looks like I have to grin and bear it

As long as you know it's "D" when you go to do things with that computer it's perfectly fine.