Unfortunately I can't post the link but I found the manual online if you don't have it, I had a skim through it and even though it states on page 80 that
"Other types of digital video, such as PC video
provided by some computers, must be decoded by the
source device and sent to the TV as analog video, S-video,
or DVI/HDMI video."
However I think you might be able to get it to work by getting your PC to send an MPEG2 stream out of the firewire, I've done something similar with DV through Premiere or Windows movie maker, it usually wants to talk to a camera but you can trick it by telling it that the device is uncontrollable sometimes it works.
Like I said I've only done this with standard def DV, the manual states that it can't decode an SD DV stream which seems kind of odd (maybe a licensing issue?) but I think you need to trick the system into thinking that it's connected to an HDV camera and stream it that way.
I've just realised that I've been under the impression that you want to play videos on the TV from the PC but now I'm thinking you may want to use the TV as an extended desktop. If that's the case ignore everything I've suggested so far as it's no use. I don't want to sound like a smartass but I think it might be time for you to get a new video card with DVI or HDMI output, I'm not sure but I'm afraid going VGA to HDMI/DVI may result in the PC not seeing the TV, I'm sure some other people here know better than me though. Does your video card really not have DVI or are you already using that for your computer monitor?