Darren,
Like the others, I too would like to express my sadness and deepest condolences on the loss of your father.
I am aware of an application that worked with older NT / Win2K servers. It looks like it has been updated through Vista.
http://home.eunet.no/pnordahl/ntpasswd/ - You might want to be careful to verify operation with Win7, as the program boots as a Linux app which then adjusts the windows SAM entries with new hashes. I have never had any issues, but I'm not using Windows 7. As another post mentioned, if the data is encrypted, which you will discover pretty quickly using the secondary drive suggestion, then DO NOT use this solution.
You might also want to try Microsoft support, given the situation, I would think they would be willing to go above and beyond to help you.
Our thoughts and prayers are with you.