One of my computers has a BIOS from 1998 and FreeBSD has always worked fine from 5.x to 7.2.
If it's a BIOS issue, you could always try to find an update.
Also, get 7.2 or wait 3 weeks or so for 8.0.
If you do try 7.2, note that it has a booting problem on *some* computers. But you can easily solve it by doing what's mentioned in the
errata (it's the first open issue).
If this still doesn't work, you may have a problem with the drive itself. Then do a
DOS partition or floppy install.