Strange PC Boot Problem

I received a phone call from Colin last night to say that his newly upgraded PC wasn't displaying a picture on his monitor when he took it back upstairs. I talked him through a couple of checks before agreeing to pop in and have a look.

It turns out that once connected to the monitor upstairs it wasn't posting. It wasn't a problem with the monitor because his other PC was working fine on it. After opening it up and noticing that everything seemed to be running okay he mentioned the fact that his mousepad was lit up without the PC switched on. I incorrectly suggested that shouldn't happen and unplugged his mouse and keyboard. The PC then proceeded to boot fine. It transpires that the little green adapter he was using to convert his usb mouse into a ps2 mouse was preventing the system from booting. I plugged the mouse into a normal usb port and the PC is working fine.

It's another little thing I can suggest when people say their PC won't boot.

Oh, and I took the opportunity this morning to open up my PC and turn the hdd led connector the right way round so I have led indication of hdd activity. Unfortunately, the power led connector on my case is 3 pin and is only 2 pin on the motherboard so I still have no visible led indication that the PC is switched on.