Question : Problem: Problem upgrading from Athlon 3800+ to Athlon X2 4200+

OS: Windows XP SP2 (*NOT* the 64-bit version)
Motherboard: ASUS A8N E
RAM: Kingston PC3200 CL3 (two sticks, 1 GB each)
Original CPU: AMD Athlon 64 3800+
New CPU: AMD Athlon 64 X2 4200+
BIOS, chipset drivers and other hardware drivers are up to date, as of one week ago.

My system, with the original CPU noted above, runs without issue. When I replace the original CPU with the new CPU, I experience an array of issues:

With both sticks of RAM and new CPU installed:
System sometimes hangs at POST. When it successfully boots, Windows will hang at random. It's not heat related, I'm certain of that.

With *either* RAM stick installed singly, and new CPU installed, the following issues occur:
BSOD on boot. (This occurred consistently until I turned DEP off in boot.ini)
BSOD during driver loading or after login. Error codes are not consistent, but include 0x7F, 0x0A and 0x8E (the last in win32k.sys).
On a successful boot, Windows goes through the WGA thing and asks me to verify my install.

This is the first time that I've tried to upgrade from a single-core to a dual-core processor, so I'm at a bit of a loss. The POST issues with both sticks of RAM installed suggests that one of them is flaky enough that the new CPU throws a fit, but Windows stopping with BSODs is making it difficult to determine which (if either) is at fault.
I know that installing a new motherboard basically necessitates a reinstall of Windows, but does the same rule of thumb apply for installing a new processor, specifically a dual-core processor where the former was single-core?

Answer : Problem: Problem upgrading from Athlon 3800+ to Athlon X2 4200+

The reason is when Windows was installed it set up  a single processor HAL (Hardware Abstraction Layer)rather than a multi processor HAL which is required for the new dual core processor.
This requires a reinstall of the operating system. Sometimes a repair install will fix this as well.