Right now I have a mac and I downloaded virtual box and want to install windows 7 on it. I bought a windows 7 CD from my campus bookstore for a really cheap price but later found out it was upgrade only. To save money, I want to know what the oldest windows operating system is that is capable of upgrading to windows 7 so I don't have to pay too much more money. Being a mac user I don't posess any other windows cds whatsoever right now. Can you please help?
The oldest version of Windows eligible for an upgrade to Windows 7 is Windows XP.
From Upgrading to Windows 7: frequently asked questions → "What's the difference between the upgrade and full versions of Windows 7?":
The oldest version of Windows that you can do an in-place upgrade (as opposed to a clean install) from, is Windows Vista. And even then it's subject to certain limitations - for instance, you cannot upgrade from 32-bit to 64-bit or vice versa, and each Windows Vista edition can only be upgraded to certain Windows 7 editions. See the this Technet article for more information.
In your case, though, since you're starting with a clean slate and don't have an existing Windows installation with programs and data you need to preserve, the version to get is Windows XP, which you can then upgrade to Windows 7 with a clean (aka "custom") install.
According to this article: http://www.pcpro.co.uk/blogs/2009/08/06/eyes-down-for-windows-7-upgrade-bingo/ The oldest version of windows you can upgrade from is Windows Vista depending on which version of windows 7 we are talking about. Different version of Windows 7 require their respective equivalents in Vista.
As long as you aren't going from 32-bit to 64 bit (you will need a clean install) or using Vista starter.