It hooks up to the video output of your cable box / satelite box.
So if your cable/satelite box supports more than 1 tv and seperate viewing then yes.
If you have to cable boxes then put the slingbox on the one she's not likely to be watching.
If you only have one cable box then she must watch what you watch.
Some cable companies provide a "mini cable box" called a Digital Transport Adapter for free, as part of their response to the federal mandate a year or so ago to switch from analog to all-digital TV signal transmission. If you get one of these devices and a splitter to split the signal from your wall outlet to deliver the signal to both the cable box and the DTA, then both of you can watch separate channels simultaneously, with, for example, the TV connected directly to the cable box and the Slingbox connected directly to the DTA.