system crash

Hi
Please forgive my etiquette mistakes, this is my first time.
My PC usually crashes about 13/15ths of the way through a Search&Destroy scan. It then tries to (incorrectly) reboot from floppy.
Another recent scan flagged up a very brief error message (that I have been unable to replicate) reporting that there was a problem scanning VirtuMondo (I think) before rapidly crashing then trying to reboot.
Another recent scan did appear to be successful, but did stop about 13/15ths of the way through, this time causing BitDefender to crash- this time not causing a reboot.
But the last few scans are prompting the crash then incorrect reboot behaviour.
The crashing started with an older version of Spybot. The behaviour described above is happening with version 1.5.2.0.
Any help with this issue would be greatly appreciated.
Many thanks
Sean

it was nothing to do with fractions.
cleaned the fan and the heatsink, & just before the cock crowed for the third time, s&d is happy again.
praise be