I can't find any confirmation that the DCR-HC20 has this feature (called analog pass-thru). I do see people claiming that the DCR-HC30 does. You may be in luck.

If you can use it for this purpose, you would likely need a 4-pin to 6-pin firewire cable and a firewire port on your PC. If you don't have these things, maybe you would want to check around the web to see if your camera is indeed capable.