It is certainly possible for a keyboard / mouse to crash a pc. It's also possible for various usb devices to do the same.
If the ps2 devices work ok then you have probably found the cause of the crashes, but you won't know if it's the pc's usb setup or the kb / mouse that's at fault.
Does the usb kit work if you uninstall the Logitech and Labtec drivers and let windows use it's default drivers? (you won't get all the bells and whistles working, but it would point to a driver issue if the windows default drivers don't crash it)