OK so i've diagnosed the problem with my sisters PC as being faulty RAM.
I've borrowed RAM from a working PC & it is now booting OK. However when I get to the logon screen the mouse/keyboard are both dead.
The keyboard works fine as I have used it to navigate the BIOS.
From what i can tell I may need to disable/enable legacy USB in the BIOS. Not sure where this is as I can't see it perhaps I'm looking for the wrong thing..
PC - DELL Dimension 5150 running XP