What do you mean "Windows 7 based"? Virtually any modern 6502 assembler will run on Windows 7... unless you mean you're looking for a GUI tool, in which case, dude, learn the command line. Every programmer should become intimately familiar with basic command line tools. They may not seem user-friendly, but the command line is far more powerful than GUIs for certain tasks. GUIs are far more powerful than the command line for other tasks. Use the right tool for the job.