"physical memory dump complete"

had the pc on this am and did a spyware scan on avg anti-spy. took ages (1h30mins, when i think it normally takes around an hour or so), came up with nothing, and while the anti spy was running, firefox ran v slowly, as did itunes which had been on overnight. . came back to the pc about an hour or so after and it's beeping with a blue screen saying 'memory dump complete'. unfortunately, being a total noob, i neglected to write the details of what was on the screen. i have since tried to restart and it says i need the windows set up disks, which i don't have as the OS was installed when i bought the PC a few yrs ago. any ideas?
my wifes ipod was connected to the PC the time (if that makes any difference), and the PC wouldn't connect with the ipod when the anti spy scan was running.
also, one of the pads for ensuring the PC stays level has popped off and as a result the PC was not level at the time (i read another message on here that suggested one of the problems could be overheating and wondered if that could have affected the fan?).

the message i get at startup is
"windows could not start because the following file is missing or corrupt; system32\drivers\ntfs.sys.
you can attempt to repair this file by starting Windows Setup using the original Setup CD-ROM.
Select 'r' at the first screen to select repair"

i was running with comodo firewall and avg antivirus (both free downloads from majorgeeks.com). OS is XP.
any ideas what to do without the OS? thanks again