The number of hardware changes allowed depends on the type of machine actually. Dockable machines like laptops have a higher number because you don't want it to have to redo the product activation every time you plug it in.

Article on XP activation

It's no big deal about installing a previously registered copy of XP on a new machine if you're replacing the old one. If it does fail and say that you've already activated it then you just need to phone up the help line and they'll issue you a telephone unlocking code. Unless you're doing hundreds of installs in a very short space of time I doubt they'll care.

It's not a OEM preinstalled copy of Windows is it?