If you want an exact copy, you can use the utility that comes with retail HDs or an imaging program like Ghost or DriveImage. You said different OS though, so I'm guessing you're going from like a 98 or 2000 system to an XP system or something like that. In that case, you can't get all your programs transferred, but set up the new system (install all software you used to use on the old box), then run the XP Files & Settings Transfer Wizard. It'll make a disk for the old box to compress all your files & settings, then you can burn them to a CD or transfer over a network, or put the old HD in the new system and transfer that way.