Just so you know about how to move a hard disk with XP proper, here's the google on that -> http://www.google.com/search?hl=en&q=XP+CHANGE+MOTHERBOARD+SAVE+OS&btnG=Google+Search
"a particular software program" should have a company behind it so why not call them for an update?
Have a Windows XP computer dedicated to a particular software program which will not run under Vista. Computer was slowing down with the probable problem being caps going bad on the mobo. Bought a cheapie Compaq with Vista and tried to replace HD with HD from old computer, thus having new machine operate under XP and not having to reload all the software. Mucho problems. First I had to re-register windows and then the time clock card I had installed was not being recognised. New computer was also short of old fashioned PCI slots so it is going back.
Before I run into more problems, what do I need to do to swith out the HD in a new Vista system and replace it with the existing HD with XP and other software loaded? Clearly the MS/Compaq combo I bought makes it difficult if not impossible. Machine will only need reasonable speed and 3 GB RAM, plus room for a wireless network card, the time clock card and an LPT card since the program does not recognise USB. Thanks.