I can't find the vid online, but Will Ferrell played the cowbell with Queens of the Stone Age when he hosted a couple seasons ago. So maybe you were thinking of both performances and got them mixed up? Either way, the QOTSA performance was hilarious. I'd look for that too.
I'm posting it since my brother didn't, which may mean he doesn't know about it. I'll find out soon enough! http://theslip.nin.com/ Anybody listen? Like? Dislike? Ever talk about music? Yeah, me neither.