You will have to Buy MASM or TASM i don't know if a trial verson of either of them is avalable....
There is a way to get MASM for free, Microsoft added it to one of their VC++ Service Packs. You can download that and, before it installs, copy ML.exe from your temp directory. You don't even need VC++.

Instructions on:
http://users.easystreet.com/jkirwan/pctools.html

I use NASM myself, and I think it is a very nice program. You can get it at
http://nasm.sf.net