So there's no flat out limit thought of how many times you can reinstall then right?
Reactivate online: Yes
Once you hit the online activation limit, you will need to call (toll free) Microsoft's product activation line (phone number given if necessary). Activating by phone takes about 5-6 minutes, and you usually don't have to talk with a person either. The automated system actually does a nice job of getting you activated and on your way.
You can reinstall and reactivate as many times as you need to, so long as you are installing the OEM copy on the same computer you originally installed it on.