Windows CE is a 32 bits OS since day one, so it makes sense tools shipped with PB, compiler, linker, assembler and etc, are for targeting to 32 bits system.
But occasionally, if you are developing x86 based system and especially working on some boot code, such as boot sector of BIOSLOADER, that will be a problem.

Normally, as PB provides the prebuilt boot sector image but if you ever need to rebuilt it, what should you do? You may say as it's an x86, perhaps you can use VS or Windows SDK to build it. But unfortunately, today's desktop Windows tool chains are also 32 or even 64 bits only, you need to find something older. VC++ 6.0, but how can you find one?

This Website http://thestarman.pcministry.com/asm/masm.htm arranges some useful resources. Basically, you need 2 thing, the 16 bits MASM and 16 bits linker.
Just make it even easier for you

1. Download http://download.microsoft.com/download/vb60ent/Update/6/W9X2KXP/EN-US/vcpp5.exe for Assembler (MASM).
2. Download http://download.microsoft.com/download/vc15/Update/1/WIN98/EN-US/Lnk563.exe for the Linker.
3. And then just extract the archives and what you need is ml.exe, ml.err and link.exe