No mouse, keyboard on fresh install

Trying to install Win 7 Home Premium 32 bit on a 60Gb partition of SSD (the other partition has Win 8.1 installed and working). Windows loads the files off the disk, logon screen with mouse pointer comes on, but no keyboard or mouse function. Both are USB devices connected through the PS2 ports. Tried a mouse on the front panel, same result. All works fine when I boot to Win 8.1 using the same hardware.

Previously I had Win 7 64 bit installed on the same partition, I wanted to use the 32-bit version, so I reformatted the partition. The first attempt installed OK, but on rebooting I lost the mouse and keyboard. The second attempt lost the mouse at the install login screen.

I suppose it is something in the BIOS, but how do I fix it without messing up the Win 8.1 install??
Thanks

How is a USB device connected through PS2 ports? Try using either a PS2 mouse/keyboard connected via a PS2 port, or a USB mouse and keyboard connected via a USB2 port.

USB keyboard and mouse are connected through adapter plugs to PS2 ports. Don't have a PS2 keyboard or mouse around. This arrangement worked when initially installing Win7. Connecting to USB ports didn't work - no lights, no power, no movement, no nuttin'.

All I can find in the BIOS is "USB Support - Disabled: All USB devices are disabled before the OS boot process completes
Full initial: All USB devices are functional in the OS and during POST
Partial initial: Part of the USB devices are disabled before the OS boot
process completes"
The "partial" is the default. I tried the others, to no effect. The mouse works during the BIOS initialization, to select items. but not when Win 7 is installed. When Win 7 boots from the installation disk it gets as far as the language screen, and can't go further.

In addition, I'd work through these steps which resolve 90% of install problems: Troubleshoot Windows 7 Installation Failures - Windows 7 Help Forums

Resetting the BIOS to defaults may help. Since the OS installed correctly once something may have slipped. It's also the chance to see if you have the preferred AHCI setting.

I am reluctant to play around too much with the BIOS, for fear of messing up the Windows 8 boot, which continues to work fine. I did find that unplugging allows the mouse to work on booting into Win7. If I go into Win8 first, I have to unplug to get Win7 to work. Restart and shutdown from Win8 do not allow the mouse or keyboard to work on rebooting to Win 7.