How much of a hassle you get from MS may depend on the winXP license you have. If it is an OEM license, they have the "right" to tell you tough luck, buy a new license, since the OEM license usually says something like you can only use that license on the exact same computer you originally used it on. The other licenses including retail and upgrade are a bit more flexible, however if the OS detects that more than a couple of bits of hardware have been changed, you'll still have to phone MS to activate the new installation.
In practice, MS has let me swap hardware 4 times on my "upgrade" winXP Home license and twice on my OEM XP license. So they're not worried about home users upgrading their computer so much as they're worried about the big abusers.
And if you get someone on the phone who won't give you the new activation code, thank them, hang up, and call back a few hours later because the next person might help you out.