Older versions of Windows used different HALs for ACPI/non-ACPI, UP/MP. This is (AFAICT) no longer the case. Where you're likely to run into problems is with mass-storage controllers. Other than that, it'll probably just work.

It's all moot for me anyway, because all of the PCs I care about are virtual, so I can migrate those to new hardware trivially.

Edit: Oh, and although I've not used RedHat for many years, the hate's never left me...