Well, it doesn't just have to be movies. At work if I'm feeling low (or if the noise from the call centre is just TOO DARNED LOUD *ahem*) I'll normally bung my iPod on, either for music or for Doctor Who Big Finish dramas or for classic radio comedy (ISIHAC, The Goons, H2G2...). Or indeed modern radio comedy - can't beat The Now Show for cheering up.
At home, if I want to watch a movie for cheering up purposes, one of the Curtis/Grant romcoms normally cheers me up (despite the fact that the last time I watched Love, Actually, I got a bit tearful for no good reason), as (despite the subject matter) does Titanic.
Also, I enjoy catching up on Chuck, Supernatural, and any other ongoing programme (yay! for the recent return of FlashForward, as well as the imminent returns of Ashes to Ashes, Doctor Who + DWConfidential, and the one-off Jonathan Creek which I'm reliably informed features not only the rather lovely Sheridan Smith but also Paul McGann, so I'm triply looking forward to that one...too bad it's just a one-off...).
Although normally if I want a laugh, QI, Mock the Week, or Have I Got News For You normally guarantee a few good giggles. And increasingly, anything with David Mitchell, although I don't exactly have shedloads of his stuff. Might borrow the Peep Show DVDs from Tim some time.
Recently I cheered myself up by watching all of Crime Traveller in a week. I enjoyed that. Is there Crime Traveller fanfic? An LJ comm? There should be. *goes off to explore*