One of the most popular plugins that tons of forums run (VBSEO) has been compromised, and many forums will be having errors about Centiyo, such as "Visiting This Site May Harm Your Computer". It's caused by VBSEO.
Disable, then update VBSEO. Then if you're a webmaster, go into your Vbulletin templates and remove any code about it from the HEADER part of your template, on ALL styles. Search your templates for "Centiyo".
Tweak3D users: I will be updating VBSEO now, which may explain all these problems for the next few hours.
Changing your password is recommended on any forums running Vbulletin.