You can, but Windows needs installing on the specific machine hardware. A drive installed on a different computer will not work properly if at all.
You could buy a new (blank) hard drive and install windows to it, assuming the machine is fast enough and has enough RAM for it to work properly on Win 7.
(4GB RAM is really a practical minimum for it to run smoothly).
But, if you cannot change the BIOS settings to start from an install disc, that's still not really practical.