Start Up - Sticks At Usernames Screen

System has 2 hard drives in which boot independantly, ones in 32 bit windows vista, the other 64 bit windows vista home premium- only reason for 2 is that when machine was upgraded last year (which essentially meant almost everything was replaced!) there was data on original hard drive so pc builders put original hard drive in on dual boot so I could access it all with the intention of removing op system and turning it to a storage drive when finished (never got round to it though). It automatically boots to the 64 bit drive unless overridden at start up. The problem is with the 64 bit hard drive boot.

Yesterday all was working fine, only thing done on machine yesterday was check facebook, download emails and run a full virus scan (Kaspersky). Shut down after scan had completed this morning (ran overnight, had halted at about 80% because machine put itself to sleep too soon so completed this morning) and restarted. Appears to start okay, gets to the screen with the 3 user images on it and stops. So I have the screen with the 3 usernames showing, 2 of these users have passwords (mine and hubby's) the 3rd has no password (kids account - rarely used account) but clicking on anything does nothing - it should bring up the log in screen but nothing happens. The images do become highlighted but the log on does not appear. Same goes for the accesibility button on the bottom left of the screen and the shut down options on the bottom right - just no response.

The hard drive activity light is on and flickering though, have left the machine for up to half an hour and the light is still flickering but everything is still unresponsive. Trying to go into safe mode does exactly the same - stops at the user names screen and is unresponsive. Have tried last known good and that didn't change anything.