My guestbook has been hacked

Hi

My guestbook has been up and running for about 6 months now but one day it looked rather strange (you can see it on http://www.gardkarlsen.com/guestbook/). My web hosts tells me that this is a result of hacking and that I have to create a new guestbook. Is it possible to fix this in any way? Is there anything I can do in the future to prevent stuff like this happening again?

Regards
Gard
Stavanger, Norway
Http://gardkarlsen.com