So I will be building a new computer this week and I was going to buy the windows 7 upgrade from microsoft online for cheap because I am a student.
Because I won't be able to access the internet without an operating system (and I can't upgrade something I don't have), would it work if I pop in my vista backup disk ( I got with another computer) to install an OS, and then go online and purchase windows 7 to dl?
The upgrade requires you to have a previous qualifying OS installed and activated.
"would it work if I pop in my vista backup disk ( I got with another computer)"
I take it you mean an oem disk like HP, Dell, Acer, etc; or even an oem you purchased yourself. Then no you can't those are locked to the pc they came on and are not transferable and can't be activated on another system, not to mention that drivers won't work on yours.
The Windows 7 dvd is bootable.
If you are getting an oem version be sure to specify 32 bit or 64 bit as only one or the other is supplied. If you are getting the retail boxed version both disks will be included.
The download version would be an iso file which you would need to burn to make a bootable disk.