Mouse problem: PS/2 or serial port? Check serial not being shared with modemcard. Check Interrupt not set for printer port, disable SCSI and start PC to check the mouse. Does the mouse work on any other PC? Check in the device manager for IRQ settings of other units possibly occupying the serial port IRQ.
Is there a step in the BIOS to activate/de-activate the PS/2 port?
Is there a soundcard on board, check settings not to interfere.
Disable CD drive and check the mouse....