Not clear on what devices you have, you mention dvr, you mention camera so I'll cover the bases :
1.) If you have composite analog inputs on your pc, i.e. an all in wonder card, you can go out from the dvr into those and capture that way.
2.) If no analog inputs, but you have a dv video camera, you might be able to use it as an analog->digital converter by running dvr->composite in on camera->camera firewire out->pc firewire in->capture.
3.) If dv cam doesn't function as an analog->digital converter, you can record from dvr onto dv tape, then capture from tape to pc via firewire.
4.) This one will require you to spend money(~$200). Buy a analog->digital converter. This will allow you to take a composite source and output to firewire which can then be fed into your pc for capture.