iPad app to gesture control PC
Not sure whether they will do exactly what you want, but HippoRemote or Mobile Mouse should be worth a look, and both have (at least some) support for multi-touch trackpad and gestures. Both have Windows servers and should work on the iPad too.

HippoRemote actually has a dedicated profile for controlling Windows Media Centre, so unless you specifically want to use gestures, this might be the way to go.