Usually a CR2032 ...worth a try.
Make. series and model number of system or MB might help.
I have a PC that won't boot.
OK, so I followed the sticky suggestions - have removed the graphics card, all PCI cards and unplugged the power supplies to all IDE drives and the floppy.
Also removed all memory modules, but still nothing at all.
When I press the power switch the main cpu fan powers up, and then switches off again.
It seems to run for a longer time the longer I have left it alone, but still only a few seconds.
In earlier investigations, it definitely got to the point of trying to access the HDD to boot, although nothing came up on the display, so I thought it was maybe the graphics card. However even with no graphics card it won't do anything.
Earlier history is that it stopped one day, and following what seemed to prove good advice I gave it a clean inside - in particular the processor fan was clogged with dust. It worked fine for about a week after this, but is now dead.
Any suggestions gratefully received.