Win7 won't easily allow you to restore any file association back to FASMW for the second time. Just follow these steps;

1. Open new Command Prompt (as Administrator)
2. Type

prompt > ftype asmfile=YourFASMdir\FASMW.EXE %1
prompt > assoc .asm=asmfile

For example, if your FASM directory is in D:\FASMW, then it should be:

prompt > ftype asmfile=D:\FASMW\FASMW.EXE %1
prompt > assoc .asm=asmfile

"asmfile" is an alias of your own choosing.

But I don't know how to restore the icon. Involves tweaking the registry.

To delete any previous associations, just use null

prompt > ftype asmfile=
prompt > assoc .asm=

I think there're better solutions but this should do.