We had a thread on this the other day but I can't find it. Basically you can reinstall an upgrade version on a clean drive if it has been installed and activated earlier. You will have to call them and your computer will print out a list of numbers which you read to them and they will read you some numbers back.
If it is not an upgrade version but a full retail, you can install it again with not problems, as long as you only have it installed on one computer.