You can google quite a bit of guides to do this. The easiest way to try (and has worked 1 of 2 attempts for me) is to not enter the serial number at installation and uncheck the "automatically register online after startup" or something to that effect. You then can manually activate the serial after install, and voila! it works.
The other install (I don't have the link handy) requires a patch download that is really easy and then allows you to activate. Either way is very easy.
If you have a newer HDD or SSD, don't forget to install Win7 with AHCI enabled, or you have to do a registry edit to activate it later (That was me!).
Edit: I do have valid copies of Vista, but wanted clean installs for both computers, on brand-new drives.