Was there a code when it said boot disk error? if so then google the code and see what it says. Boot disk errors can occur for many reasons,don't go messing in the BIOS unless you have to.
firstly, if your computer has a floppy disk drive, check you haven't got a floppy in there - also check you haven't got a cd in the cd drive or a usb pen in a usb drive - sometimes BIOSes are set to boot from somewhere other than the hard drive first - more often than not the floppy drive if you have one - if you haven't got any of those in then you need to investigate further - it's possible your hard drive has failed - but there are other possible solutions
there are no disc etc.. went round to check tonight..however PC booted up OK so have started a full virus scan... she is saying she gets this sometimes but then will fail - will do a full system tidy etc when I get a chance
The fact that it has booted up OK today I hope is a good sign !!