The remarkable thing about processors nowadays is that you can dynamically change the speed of the processor by changing the power going to it. In the old days, it was always running at a particular speed, especially in desktops.
My guess it's showing as 1.5 in CPU-Z because you're not drawing much usage from the CPU. Try moving the mouse or open an application to see if the usage spikes momentarily.
I think the 790 Mhz may be referring to the speed of the RAM. Does it say Intel Core 2 Duo 2.4, 790Mhz, or is that reference next to how much RAM you have?
If not, I'm not sure why there's so much disparity between the two gauges. Do you have the latest bootcamp drivers installed? I remember with my old AMD processor, I had to install a driver in order for XP to "recognize" it, but I would think that shouldn't be a problem at all for the C2D.