technically yes, but after many years i have never had an issue. The first few times you reinstall it will register online no issues at all. After that you have to call in takes about 2 min. When you call tell them whatever component you are replacing is to due to faulty parts or if it's just a a reformat tell them you were surfing some real awesome porn and got a virus, that shuts them up real quick.
my oem xp is in it's third build with a couple hardware upgrades on each, compounded by clean installs about 2-3 times a years when experiments go bad. Did that for about 9 years. been playing with win 7 for the last few months though and it rocks.
The license agreement states that an OEM copy is tied to the original computer, however you can do as many upgrades and repairs as you like, you just can't transfer it to a new machine.
I had occasions when an old motherboard has died and since you could no longer get anything that crap I've had to change the motherboard, RAM, CPU, PSU and GPU, but technically it's still the same machine, it's just "repaired".
Depend on how often you plan reinstall or change HW and how much you like calling Microsoft. For me i am often doing some changes in my system every 3 months and really don't like spending 15-30 min on phone.
Once i got really p**s off when i for my father installed oem XP and after connecting 2nd hdd to make backup it start saying that hardware on computer changed and required new activation.