Well your xp x64 windows can still run 32 bit code so you wont need a special assembler for x64 just any x86 one will do.
The one i used is MASM32
http://www.masm32.com/
the website is bad on the eyes, but its meant be compatible with MASM. You can use any editor to write the source and then just assemble it with MASM32. Its still only 32-bit, but unless you really need to program in 64 bits it is all you will need.
edit:- there is also NASM which is good too.
http://sourceforge.net/projects/nasm