I've been a member for a while now and I have never seen a popup asking to "rate this forum."
To be on the safe side, I'd run an extensive virus scan on your machine just to be careful. Malware is rampant out there, it can't hurt to be too safe.
Also, because it specifically said 'rate this forum
' it could just be some kind of benign marketing software running in the background that you didn't even know you downloaded one day who-knows-when. Every few months they're trying to get you to interact with them so they can expand their market and databases... that's how they make money. Benign, maybe, but annoying as hell to be sure.
Whether it's a virus/malware, a marketing trojan or it turns out there really is a legitimate 'rate this forum' popup, the only thing you lose by running a full virus scan is... um... nothing