If it's 98/ME you should be able to run the same HD. Might be buggy though. Given the chance I'd reformat anyway just to get a good clean install. If you get it going "o.k." consider backing up your important files on removable media then reformat.
If it's win XP you will probably need to go through Windows Product Activation. This may even require a telephone call to Microsoft.
If it's win XP you will probably need to go through Windows Product Activation. This may even require a telephone call to Microsoft.