I am really interested in assembly language and I want to learn about how exe files work how dlls run etc... and I have an idea of writing an application to decompile an exe to assembly code since i am not a very good assembly programmer and with the lack of knowledge of the inner working of exe I couldn't do it. Since I can read an exe in hex i think it is not impossible but I don't know how to write my own program. Any resources or any help would be appreciated.
I think you're looking for a disassembler not a decompiler. IDA pro seems to be popular and you can download an older version for free at http://www.hex-rays.com/idapro/idadownfreeware.htm