Desktops have a battery too
Its a little 3 volt cr2032 in most cases.
Look for it disconnect the power(at the power supply) remove it and short(if they are at 1-2 move it to 2-3 for a few seconds then back) the pins called clr_rtc or clr_cmos.
Just to give you an idea of the battery look. Its the KTS one, the red box is the jumper location for a 2 pin only board. more boards have 3, but you get the idea.
If that fails, if you have onbaord video connect to it and look in the bios for an option to boot from PCI-E and not onboard or IGP.