Cause you mentioned that when you enter the bios and change nothing and leave, then it works... I guess that some of your Bios settings are erased when you start (check cmos battery status) or that your cmos is trying to do an autoconfig and fails during POST, but not during a warm boot after you checked the bios. Only some thoughts...
btw: if your PS/2 port is bad... disable it in cmos and use a serial mouse on one of your serial ports (com1, com2) in DOS. No need to install a mouse card

It seems that Guus typed faster then me... sorry. I was in the chatroom while posting this. greetz to Guus