if you type security tool in the ( search the answerbank ) box at the top of the screen
you will see what other people have posted.
when i had it, i had a computer repair man sort it out, had to take the hard drive out of my laptop think it cost £40
Thank you all for your help. All is well again, but in the end the only 'free' way I managed to get rid of 'Security Tool' was to use the 'recovery disk' that came with my Advent computer which I bought over 8 years ago and has hardly ever been off since.