Any retail version of Windows can be installed/reinstalled as many times as you want but on one computer at a time. If you're been running on an old home made computer, before installing it on a new computer just format the old one. You won't have any troubles activating it on a new computer.
OEM versions have to be installed on one computer & remain on that computer.
There are also corporate versions which you can install on many computers at the same time (even without activation in some cases) but they cost a fair share of money.