Well activation has a time out. So chances are if you activate on PC1 today, and then build a new one 5 years from now and try to activate I doubt you would have problems.
If you did, you just need to call MS. Tell them you are installing on new hardware due to a total failure of the previous PC. They will ask if that PC has been destoryed (basically you no longer are using the license on that PC), you say "yes". They give you a code to put in, and you are done.
Of course if the next PC won't be for 5 years or more then XP will likely no longer be used anyway, so it is a moot point. OEM is the way to go. The biggest difference is no tech support. Even with retail I think you only get limited free tech support anyway.