Windows is generic until it configures itself to each motherboard. Each installation is specific to a particular motherboard.
With harddrives so cheap these days, what I would suggest is picking up a new harddrive also. Then, slave the old drives and extract from them any pertinent data.
However, it would be an excellent idea to make backups (burn some CDs or DVDs) with all data that you cannot afford to lose. It's also a good idea to have any programs and drivers you want on disk and ready to go.