Jefro and Thorkelljarl, thanks for your replies.
When booting with keyboard and mouse unplugged, and then plugging them in a few seconds after the login screen came up, syslog shows:
[ 10.078380] usb 2-1.3: >new full-speed USB device number 5 using ehci_hcd
[ 10.170668] usb 2-1.3: >New USB device found, idVendor=045e, idProduct=001c
[ 10.170671] usb 2-1.3: >New USB device strings: Mfr=0, Product=0, SerialNumber=0
[ 10.170936] hub 2-1.3:1.0: >USB hub found
[ 10.171156] hub 2-1.3:1.0: >3 ports detected
[ 10.442025] usb 2-1.3.1: >new low-speed USB device number 6 using ehci_hcd
[ 10.544309] usb 2-1.3.1: >New USB device found, idVendor=045e, idProduct=001d
[ 10.544313] usb 2-1.3.1: >New USB device strings: Mfr=0, Product=1, SerialNumber=0
[ 10.544315] usb 2-1.3.1: >Product: Microsoft Natural Keyboard Pro
[ 15.193387] usb 2-1.4: >new low-speed USB device number 7 using ehci_hcd
[ 15.291411] usb 2-1.4: >New USB device found, idVendor=046d, idProduct=c050
[ 15.291414] usb 2-1.4: >New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 15.291415] usb 2-1.4: >Product: USB-PS/2 Optical Mouse
[ 15.291416] usb 2-1.4: >Manufacturer: Logitech
When disconnecting, this event is also logged. I tried just connecting the keyboard, and not the mouse. Same result.
The Alt-SysRq keys are also not working.
In the BIOS legacy USB devices are enabled.
When booting with the keyboard connected, about halfway pressing the numlock key stops toggling the light. Unfortunately even without "quiet splash" in the kernel parameters, the screen stays blank for about 7 seconds, and then the login screen is shown.
Yesterday I tried to install Ubuntu, but after a successfull install the reboot resulted in grub not finding files it needed.
Today I tried Fedora: no problem. Keyboard and mouse are functional. So it doesn't seem to be a hardware problem.
I just tried Linux Mint 15 Cinnamon 64 bit, and to my surprise this one is working!
Why doesn't it work in Linux Mint 16 Cinnamon 64 bit then? I have installed that about a dozen times the past days, everytime with the same result.
Unfortunately in the linux Mint forum I am getting no reply...