[MadDog]

Or, you can:

1) Install W7 without putting in the CD key.
2) Upgrade from W7 to W7, this time putting in the CD key.
3) Activate W7 as normal.

This was the way to do it for Vista, and should work also for W7.

Got this from a tech website:

click Start, type: CMD
Right click CMD, click ‘Run as administrator’
At the command prompt, type, regedit
Go to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersionsetup/OOBE/mediabootinstall
change its value to 0
Go back to the Command Prompt, type the following:
slmgr /rearm
Restart the pc and use the activate windows to re-enter your product key