Hi Arif Deshmukh,
Glad to see you.. So you want a WIN based assembler, But what do you exactly mean when you say a WINDOWS based assembler...
1. Do you want a WIN32 console based Assembler instead of a DOS Real mode or protected mode assembler.
-But Remember in console mode things would look same to a programmer wheather WIN32 or protected or Real mode.
You can download ASEM-51 by W.W. Heiz it has all the three version packed together i.e. (i) DOS Real mode. (ii) Protected mode. (iii) WIN32 console mode.
Here is the link : http://plit.de/asem-51/download.htm

2. Do you want a Graphical User Interface [ GUI ] for writing programs I mean syntax highlighting nice IDE [ Integrated Development Enviourment ]. If yes then download MIDE-51 studio by K. Worapoht available here : http://www.opcube.com/home.html

MIDE Studio and ASEM-51 go hand in hand Actually when you use them the MIDE studio provides the GUI stuff and ASEM-51 is the undelying assembler.

Both the softwares listed above are udner GPL and are absolutely free. Let me know if my post helped you.

Regards,
Prahlad Purohit