There could be a couple of problems:
a) If you've connected from the SCART socket on the Freeview box to the S-video socket on the TV card, I presume you've used some sort of adaptor. Could you double-check that the adaptor is of the signal OUT variant or, if a switched type, is physically switched to OUT. If it's an IN variant, or switched to IN, that will result in the symptoms you describe.
b) You /may/ need to select the S-video format at the SCART socket. Go into the Freeview set-up menu and see if there's a video output format setting. It may give you a choice of Composite (could alternatively be called Video or PAL), S-video or possibly even RGB. If there's an S-video option, make sure that's selected. HTH.