How do I clean my hard drive?

My husband gave me his old Macbook Pro (he upgraded to one with more memory) and I need to get all his files off of it. I am a PC gal so this is all new to me. I've googled the heck out of it and nothing I can find is working.

Specifics:
Macbook Pro running Mac OS X Lion 10.7.3
Currently it has 54.26 GB free out of 159.18 GB

I created a new Admin account and deleted his user account but it didn't delete his files or data.

I went to Disk Utility, clicked on Macintosh HD, clicked erase, but the buttons that say SECURITY OPTIONS and ERASE are grayed out and won't let me click them.

We do not have any start up disks for the computer (husband claims they didn't come with his computer, I have no idea.) What can I do to clean this thing off so I can get my PC info migrated onto it ASAP?

Thank you so much!
Emily

How did you delete his account without deleting his files?

Alos you dont have to wipe the hard drive to create a clean account, but first, are his accounts backed p