"No user specified. If you followed a valid link, please notify the administrator."

That's from the PC database, as far as I know, and usually means you followed an invalid link -- for instance, to the profile of someone who no longer has an account here. That message has probably been part of the vBulletin software from way back. I'm not sure just why it suggests notifying an administrator; I can't think of too many situations where that would make any difference.

"PC is not responding. Recover web page."

I've never seen that one. I'd guess that (a.) it's coming from your browser, not PC and (b.) your browser is different from mine.