bios rebooting - cannot load WindowsXP

Model dxp061 (410 series)
Bios 2.5.1 (I just downloaded 2.5.3 on another pc but I'm not sure howto install it)

I have an XP home SP2 system that has not had any significant problems. However - I now cannot boot. WindowsXP - I get the bluse DELL logo, then the AHCI page which says AHCI loaded... then a reboot to Blue Dell page or it just hangs.
I ran hardware diags and passed. I checked bios settings and they seem ok. I can boot from CD - but not from the HD.

I was thinking I should install the latest BIOS (253) but am not sure how how to boot and install the bios upgrade (an exe file).
Can anyone tell me how to install the bios when I cant boot?
Anything else I can do?

thanks