If you install XP using the current system, and activate it on that system, there shouldn't be any real problems so long as you aren't doing a lot of hardware changes. Probably not much of a problem with an eMachines system.
You shouldn't need to reactivate each time, unless you've made some significant hardware changes, though you may need to spend some time installing all the updates that have come in since the last time. And hardware changes are about the only possible issue I can see you having. If, for example, you replaced the motherboard on the system, both Vista and XP would likely need to be reactivated if not reinstalled. If the hardware configuration remains fairly static, then there shouldn't be any problems.
What problems might be associated with swapping master hardrives? If I have an eMachine that currently is running with a HD that has Windows Vista, is there any reason that I cannot take it out and put in another HD that will have Windows XP Home running (I will Install Windows XP and ?activate? it for use in this computer)? I know that I could partition the current HD and set it up for dual booting but there are reasons why I do not want to go in this direction. This swapping of HD?s would happen once or twice a year.
A corollary question would be: would I have to re-activate the Windows OS each time I made the swap and if so what problems might be associated with that?
Intel's P4 631 3.0GHz 64 bit processor with Hyperthreading. 1024MB DDR2 memory and a 160GB, 7200RPM hard drive.