Can anyone recommend a free keystroke logger out of the many available that
I can use to find out who and when (that will be my lodger, her boyfriend or
her mates whilst I am at work) is using my computer.

Put a password into the BIOS.

Its still possible to work around, but a whole lot harder to crack than
a windows password - and it`ll also prevent them from using something
like Knoppix.

Most BIOSs can still be hacked fairly easily, but if the machine isn`t
already in a booted-up state, I think it pretty much knackers them.