FWIW, here's how I connected my NTL (now Virgin) cable box:

Scart out from the cable box to...

Scart to svideo adaptor (it's a scart plug with three phono plugs coming out from it - video, left audio, right audio), the plug is stamped OUT (if it had been stamped IN it would have been an input adaptor)...

Triple phono lead to the...

Input sockets of a Hauppauge PVR 150 MCE analogue tuner card.  There might be better versions of the card, and there might be better manutacturers - but that's a commonly used card.  The 'MCE' is important.  That means it comes with an MCE compatible remote and an IR blaster to change your channels for you.

The connection from your PC to your TV will depend on your equipment, but you have all that already so you should be able to work something out,

When it comes to configuring MCE:

- The config process will find an Electronic Program Guide (EPG) for you

- I had to manually enter a channel number because the scanning process didn't find it.  My NTL box is on channel 52.

Getting the IR blaster to work is a major pain.  Because of the way the STB works, the default set up will not change channels correctly if the channel number contains a repeated character.  So 101 will change fine, but 444 definitely won't.

You therefore have to 'fool' the blaster into sending the correct characters.  See: http://thegreenbutton.com/forums/1/142366/ShowThread.aspx for a discussion on how to do this.  Read through to the end because there's a couple of red herrings.

Finally, if you are expecting to be able to use MCE to watch and record both Freeview and Virgin you are going to be disappointed.  MCE can't cope with that.  To get around the problem I use a different media centre (GB-PVR) to watch and record Freeview.

Hope that helps,

Andy