You have to make sure whatever ISO you download matches the exact Windows version of the license key; there’s a whole bunch of different versions (Ultimate/Pro/Volume/International/OEM/etc etc). For that reason, I’d look at the documentation that came with the volume license - it’ll probably tell you where to get the correct
You don’t want to go through all the trouble of installing Windows 7 and then find out you installed the 32-bit International Home version and your license is for the 64-bit US OEM Volume license, because you’ll have to start over with the right ISO.
At least, that’s how it used to be. I guess there’s a small chance they’ve changed it with Windows 7, but I wouldn’t bet on it.