I was doing some development work with a Microsoft product (I think it was 'Front Page'), and after about 3 installs I had to start calling Microsoft to activate the product over the phone. Talk about a pain in the....
That's how I found out about this - I am taking an A+ self study and use my own computer as the test machine. I got sick of calling MS and trying to explain that I don't have my copy running on multiple machines.