As long as you've got a legal version of the Windows 7 installation files, put them on a bootable DVD, then boot from the DVD..

On the other hand, if you're looking to perform an "upgrade" install, (hopefully, you've got a compatible upgrade version of Win7), you'll need to get the password. Try Googling the "NTPassword" program to get that done. A link is below:

http://pogostick.net/~pnh/ntpasswd/

Hope this helps.

Grif