Cant get into safe mode

Hi, im trying to fix my computer, 2 days ago i was presented with a page saying your computer has been locked ploease pay £100 to international police association to have your computer unlocked, i know this is a virus so done a search and found out i need to go into safe mode to get rid, but, every time I boot up, it goes to the same page and if I hit F8 and attempt to boot up in safe-mode it gets to the starting windows bit and then shuts down and reboots normally, ending up at the same page.

Any ideas please?

the computer is running on xp (i know there is a thread with the same problem but he is running on windows 7 so the software the guy said is not for xp)

Thank you in advance