|I believe they have gotten even stricter on vista than xp, for example if you want to do a clean install of vista from an upgrade disk first you must have an upgradeable version of windows to install from, so a blank disk would mean installing xp then installing vista, while the xp upgrade installer happily accepts copied/cloned/pirated copies of win98 for upgrade compliance - no license key required (well at least mine does).|
I'm not so sure about vista but xp retail copies could be transfered from one pc to another - check the eula. Even if the eula states something similar about only running on one pc at a time don't expect to activate without a phone call to india.
When I transfered my retail copy of xp net activation stated that my copy had been "activated too many times", the automated phone prompt system spat out some similar garbage and I had to select "if you believe this is in error press x" then explain to some guy that the old mainboard was not to return and had died of cancer(bad caps) before he would give me an activation key.
Apparently asking them "but I can still use it for 30 days so I can download and burn some linux images right?" greases the wheels but I haven't tried that myself.
Of course if you don't have a proper install cd to start with (I don't think dell/hp/compac rescue cd's count) and you want a copy your probably going to have to shell out.
On top of that if you try to simply transfer the hard disk it will tell you that your hardware has changed significantly and you need to re-activate.