i got OEM xp pro with a £20 hdd for a total of less than £180 (xp home full price) off of novatech
only lets you install on 1 hardware though. if you change the cpu/mobo/hdd xp will not run anymore - also i think there is a max of 3 reinstalls
All that happens after the limited number of reinstalls is that you have to ring Microsoft and convince them that its not a new machine. Mobo is probably the significant factor. You are certainly allowed to expand and repair machines.