Why doesn't Windows XP run on a 80486 processor?
There are several reasons why Windows XP does not run on a
1. Most 486 computers simply cannot support enough memory to run
Windows XP. 64 MB is pretty much the maximum you'd ever find in a
486; 8 to 16 MB is most common.
2. The kernel in Windows XP is SMP-enabled by default. To
support SMP, Windows XP uses an instruction called
CMPXCHG8B, which is only found in Pentium and later
3. In order to allow for optimizations on-the-fly, such as using
MMX or SSE instructions, Windows XP also requires the CPUID
instruction, which only the later 486 processors had.