Did you sign a computer policy form when you started your employment, this would specify what you should and shouldnt be looking at and when of course.
Most firms nowadays have a fairly decent firewall which will stop these sort of things being looked at in the work place , even if you stumble across them in innocence, and some even keep records of how many sites like this you have tried to access in the past?
Depending on your previous work record, I would suggest you call a meeting with the company HR department to explain your actions as suggested by Peter & Whickerman, have they suspended you pending an enquiry? It may be that they are searching your PC for evidence of you looking at these sort of things before