Thanks for the reply's.

In answer to your questions, i have run anti virus, spyware etc, PC is an acer aspire t180, running windows XP, i haven't tried a system restore and haven't done/changed anything for a while.

It is only doing it on a few forums, mostly photography forums, i am using firefox and i have tried with IE and get the same.

Restarting dosen't help.

Thanks

Nick