TV says "not supported" when I plug it into my computer. Why?

I bought two Orion 24" LED-LCD HDTVs. Directly out of the box, I tried to plug one of them into my PC using a VGA connection, (my graphics adapter does not have an HDMI port.) But when I changed the TV's input mode to "PC", it said "Not Supported." I think it might just be a matter of settings, but my graphics adapter's "control center" thingy won't open--when I try to open it, it says it has encountered a problem and needs to close. Whatever. I was going to update my display drivers but the most recent driver I could find was released before I bought my computer and the guy I bought it from said all of the drivers were current, so I just didn't bother. Someone suggested to me that I should reinstall the driver, but I don't know how to do that and I'm not convinced it would do any good.

The manual for the TV says that it is capable of being used as a computer monitor, so either the manual is lying or something's wrong with my computer. I called the Orion tech support number and it was no help. If anyone can help me, I'm running Windows XP Professional, Service pack 3 with a Radeon X1600 display adapter.