ukash virus on vista

My computer has just been infected by a variant of the UKASH virus. " ASSOCIATION OF CHIEF POLICE OFFICERS - INTERNET WATCH FOUNDATION" YOUR COMPUTER HAS BEEN BEEN DOWNLOADING INDECENT IMAGES etc etc...." Asking me to pay £50 via UKASH to unlock. I have tried the usual solutions by going to safe mode but all safe mode options including "with command prompt" are blocked. I just get a small box on safe mode screen which just brings up the offending screen no other icons whatsoever Any help for me out there? Best wishes Pet