I Think, Therefore I Am in Trouble - S5.E16

Supermodel Claudia Schiffer guest stars as Gretchen, a drop-dead gorgeous new lawyer in Greg's office about whom he's having sexual fantasies. Guilt-ridden over his inappropriate thoughts—especially since it's his ""four-and-a-half-year anniversary"" with Dharma—Greg tries desperately to get Gretchen off his mind, to no avail.