There is a similar post about this issue, but mine is slightly different.
I've just built another computer and I haven't yet connected the power and reset buttons to the motherboard, but I have connected the power supply to the motherboard. However, when I plug the power supply in and flip the switch on the back, my computer actually tries to boot. It doesn't automatically turn off (like the post linked to above), instead I get some BOIS beeps (one long, two short), which according to my BIOS means I have a video error (which could be seeing as I haven't plugged a screen in yet).
What could be causing the instant boot?