Currently I'm using JNES. It seems to work great for all of the roms I've downloaded. What are some other good emulators that work well in Windows XP? I tried out Nesticle a while back, it seemed pretty good but I was running windows 98 back then.
Nestopia is the most accurate AFAIK, and it has an NTSC video filter which is really cool.
FCEUXD SP has lots of awesome debugging tools.
Those are the two I use most frequently.
Nestopia and Nintendulator are good. From what I hear If your game works on both these, chances are it will work on real hardware.
I've used JNES, I use some scrolling tricks that work on real hardware that don't work on JNES, so it's not so accurate if you are doing anything with status bars, or mmc3 irq's and changing PPU registers mid frame.