4) Cablebox with Firewire output + firewire port on PC.
It works really quite well.
Cable companies are required to offer cable boxes with firewire (usually the HD ones all come with it). However, depending on your cableco, the firewire output may or may not be encrypted. You can only connect it to your PC if it is not encrypted.
Note that the presence or absence of encryption on the Firewire output is *totally independent* from whether the data is encrypted on the cable line. The cable box decrypts the signal with its cable card, and possibly re-encrypts it for the firewire output, depending on the cableco's settings.
For me (with RCN Boston), I've found that all the extended-basic channels are sent unencrypted over the firewire output, except, paradoxially, *sometimes* the HD OTA channels. I dunno what's up with that, but my solution was to just not go through the cable box for those channels. I don't subscribe to premiums, so I don't know whether they are encrypted or not. Your mileage may of course vary, depending on provider and possibly even region.