CMOS ERROR

One of my 450 AMDK6-2 machines gave a CMOS checksum error – Hit F1 to enter setup. I found that all settings were gone so I changed the battery, cleared the bios and tried again. I now get a low battery error, a post code of 2s 1long (AMI) and a hit F1 to enter setup. The setup screen appears and it finds one drive and then locks up.
Does anyone have any ideas. I tried 3 batteries and did clear bios each time.
Thanks.