I'd take out the CMOS battery and reset the CMOS to defaults.
Unplug the PC from the power before opening the system up
Look [http://support.dell.com/support/edocs/systems/opgx260/en/index.htm#online_documentation|HERE] at the service manual - specifically the '''Battery''' section.
Remove battery and leave it out for a few minutes. Might be worth testing the battery voltage while its out. replace as required.
This may get you past the Dell screen and allow access to BIOS.