as long as its the same hardware, i think you can reinstall it again and again. i have winxp home on an hp pavillion 750, formatted it atleast 7-9 times already, no problems with the windows installation activation and updates. should your machine hardware change(significant), you might need to call microsoft for another reactivation key(at their discretion) or you may have to buy another win license.
and you cant install OEM software to another machine, its piracy, for Full Packaged Product windows(the box type) they have slightly different EULA where u can install the product on 1 desktop pc and optionally on 1 portable pc(laptop) provided that the user of the machine is just a single entity. i dunno if the EULA has changed though, but last time i checked this was it(and i think its similar to win2k pro license arrangements)