Once again, another rampant display of EULA neglect. You guys should read yer EULA's once in a while. EricB is turning blue in the mouth but sounds like no one's listening.
You're allowed to have 1 desktop installation and 1 laptop installation. That's base.
Activations depend entirely on the type of disk/license you purchased. Some disks are only good for 2 activations. Others don't care as long as you only have 2 machines registered at once. Some you can continue to use only if you call and activate. Others are completely worthless after 2-4 installations. Read your EULA. If you don't understand it, call Microsoft. They'll help you resolve your situation. Despite all the zombie hype, Microsoft does provide nice support concerning licensing issues.