The Big Bang Theory Season 1 Episode 7 : The Dumpling Paradox

Penny's promiscuous friend Christy (Brooke D'Orsay) from Nebraska is in town, and ends up sleeping with Howard. The pair takes over Penny's apartment, causing her to sleep on Leonard and Sheldon's couch for the night. With Howard gone, Penny takes his place playing Halo 3, causing Sheldon much distress as she is naturally good at it. Furthermore, Howard invites Christy to move in with him (and his mother), taking up all his time.