A number of people running Pentium 4 Prescott CPU's with older Bios versions (on some motherboards) are experiencing a failure while booting Windows after installing Service Pack 2, this is due to an issue with the machine's BIOS not installing a production level microcode update. If you have an issue after installing you should be able to boot by opening the bios and temporarily disabling the L1/2 cache in your system, however if you’re going to be installing SP2 on a P4 Prescott it is probably worth getting the latest version of the BIOS for your board from its manufacturer first.
For more details on known motherboard models with a problem see:
http://forum.aumha.org/viewtopic.php?t=7447