I purchased a 25 foot Philips coaxial cable, and am wondering why my picture is bad on my HD channels?

If it doesn't have gold connectors I would buy one with gold, its a better conductor. Beyond that you need to just get the HD box so that you get the true quality.

Maybe you have a loose connection somewhere. If not, I'd exchange it at the store where you bought it for another one.

It could be the legnth of the cable. The longer the cable, the more distance between the signal and your TV, meaning the signal has more time to break down before it reaches the TV to be put on the screen.

25 feet shouldn't be that long for a coaxial cable. It shouldn't really degrade the signal any, at least in theory. Have you tried moving the TV back to where it was before and using the old cable. Does it improve immediately when you use the old cable? I would test that out. If the old cable seems to automatically fix the issue, then its either the length of the new one, or just the shielding on the new one. So then you can try returning it, but going with a different brand next time. If the problem still persists, and the old cable still delivers good image, then perhaps you need to add in an amplifier. So use two short runs of coaxial with an amplifier linking them together (you can get a cheap amplifier from most electronics shops like a Radio Shack or online from like Amazon). Alternatively, you could call your cable company and ask them their advice on the matter. Perhaps they can send someone out and run you a longer piece with their cable. I've moved a couple of times recently and had the cable guys come and give me fresh coaxial lines each time, and they seem a lot better quality than anything I find in the stores. But that's just me, it might not be the same for you, but thought I'd mention it just incase.