WinXP won't lock you out if you overclock. Also, you can swap out a fair amount of hardware before it notices. Once you make enough system changes, however, you'll get a pop-up stating you need to reactivate it. This takes all of 15 seconds once your online, though, and then it runs fine.
It's also one way to get out of having to call in for manual activation if you're doing alot of upgrades to your system. Install all the hardware first, but don't reformat the HDDs. Start WinXP, it will notices the changes and ask to be reactivated. You do it quickly online, and then reformat/reinstall. When you finish the reinstall it's already activated with the hardware in your system, and you never have to call in