I've been working on my girlfriend's old Dimension 8200, converting it into a media system of sorts. We've upgraded the memory (a whopping 512 MB) and switched out the Nvidia GeForce2 card that came with the CPU for a DirectX9 compatible GeForceFX (EVGA 5200) with a little more memory (128) and AGP2/4 compatibility. So far, no problems. VGA and S-video to TV looked fine.

Then I got my hands on a hardware encoding TV tuner for cheap (an AverMedia) but the reception sucked from antenna and it had very poor software compatibility (as in none, for MediaPortal). After some thought about Hauppauge tuners, we decided the over-the-air analog/digital switch was coming too soon so we went with PCHDTV and a switch to Linux.

We just got the tuner and switched out the AverMedia. This is where the problems start:

We started up Windows and got the Dell boot/BIOS screen just fine.
We get the early Windows load screen just fine, but then the VGA signal dies (no signal to monitor).
However, the S-Video works! The output to TV works just fine.

I've tried:
1. Loading without the tuner card installed in the PCI slot.
2. Loading with the old Aver tuner in the PCI slot.
3. Loading with a Ubuntu Live CD.
4. Loading with the S-Video and VGA connected.
5. Loading with the VGA connected by itself.
6. Using the monitor on another CPU (works fine)
7. Browsing the BIOS settings (everything looks normal).

Lastly, there's NO internal GFX on the motherboard, so its not a question of disabling that.

Help?

Have you tried booting into safe mode, and then checking your video settings?
The VGA port obviously works, as you said you can see the Bios and other boot screens, including the early Windows screen.
Sounds to me like it's switching to a video mode that isn't supported by your monitor or something.
Did you do a reset on your monitor to make sure the problem isn't with it?
(EDIT: oops, I just see you tried the monitor with another machine, so, scratch that!)
I also see you mentioned that you used an Unbunto Live CD, and, I assume it does the same then?
If so, maybe you have some conflict in your BIOS that occurred when you inserted the PCHDTV card?
Maybe it grabbed an interrupt that was needed, and now the GFX card is using an interrupt that is a conflict with something else?
maybe not, but, maybe trying a 'reset to defaults' or to optimum defaults in the BIOS to see what happens?
All wild guesses, but, since it did start when you installed the PCHDTV card, I'm thinking it's a resource issue, that wasn't returned to the previous state when you removed the card.

Good Luck!

If you are switching to linux then why are you trying to boot into windows?

Fixed.

Turned out that the driver was corrupted. Reinstalled driver and then switched over to Ubuntu flawlessly.

Thanks!