Well, I am not surprise about IE 5 at all... Looks like a the box model issue, there is a conflict between the padding and margin I would think. (Sorry, I am mac user, so I can't really help. btw, I use this great tool also frequently...:-))
I'm a mac user too. At the end of the day I can't be too bothered trying to make the site look good for people who use outdated browsers. I might play around with the margins a bit.
Yeah, the net renderer is great and saves me from having to run Fusion.