If your virtualbox VM is hosted in a datacenter, you can usually get them to lease you the license under their SPLA.
Windows 7 you can't use SPLA - but windows server 2008 and 2012 you can.
For home use, work use, where there is no spla available - then the only other option is a corporate volume license agreement (windows 7 enterprise can be done on this)
again you said you are not a corporation so presuming you dont have that.
so in short, there is no rental option (monthly pricing) for you on windows 7, or server editions.
Your next best legit/legal option would be to sign up at microsoft as a developer, or for one of their many programs (biz spark, or somethign 'spark' they change it around ever 6 to 12 months) and you likely can get in on a FREE suite of licenses including 1 or more server edition licenses, and SQL server license etc. Its designed for start-ups to create on the windows platform by giving them the 'intro' licenses for free until you go into full production (i.e. host it and start selling something). TEchnically those developer licenses would be legit for you to use at home/office.
Hope that helps!