yes. that's 4/5 ~ 1MHz below the target FSB of 100MHz. Did you select spread spectrum modulation in the BIOS?
Anyways..the higher the temperature the higher the higher the variance will be. you might check the datasheet of the PLL about this, too.
Can a processor make windows crash?
I would consider this OK. If two speeds for that processor type were for example 450MHz and 500MHz, and the processor was running in the 460, 470, 480 ranges or perhaps above 455 or below 495 I would be concerned at be checking the settings for the CPU to make sure that you cannot get closer to the target speed. But I would consider 496MHz to be close enough to 500MHz.