Legal, yes in all likelihood. "allowed" as defined by Microsoft, no.
Microsoft's license agreements have been proven to be nearly unenforceable when it comes to things like this. About the only licensing issue that you can have a problem with is installing it on multiple machines. Anything past that, they can't really do anything to you. Any company that they sell OEM copies to can legally sell it to you, and you can legally install it. Microsoft doesn't seem to have made the contracts that are used for OEM licensing to resellers say that they can't sell it that way, but they frown on it and may use pricing tactics to help prevent it, but then that gets them in trouble with monopoly issues.
Legally, once you pay for a license for the software, you can do whatever you want with it, except install it multiple times since you only paid for one license. OEMs of course make it hard to install the OS on any other machine by not even giving you a disc, but any other OEM version disc should take the OEM license key from that machine if you want to install it on another machine later.
Microsoft COULD come after you and sue you, and they'd probably lose, but you'd go bankrupt fighting it and they aren't interested in a single person installing the OS on a different machine as long as it's only one at a time using it.