I have Windows XP, and I have had Directx 9.0c for a while, without any issues.

I just got the newest 9.0c (november 2008). Now the 1152x864 res that I've been using since it was released won't work, as the monitor goes black with the message in the middle of my screen "Cannot Display This Video Mode". Everything else functions though, as I still hear normal gameplay in the background. 600x480, 800x600, 1024x768 I tested these and they work fine, and any res beyond 1152x864 even works as well, but my monitor isn't big enough for those to be viable. 1152x864 seems to be the only one getting the shaft. If nothing really comes out of this, I can settle with playing 1024x768, but I'm bringing it up just in case someone has any ideas. Thanks.

It sounds like the new DirectX version enabled some display modes that aren't really supported by your monitor - most likely having a refresh rate higher than the one your monitor supports at that resolution. You could try using software like PowerStrip (http://www.entechtaiwan.com/util/ps.shtm) to disable these display modes.