You would need Microsoft's Virtual PC with a legitimate copy of Windows. Although with only 320MB of RAM and a 400 Mhz G3, you would be below the requirements to run it and Virtual PC was also discontinued a few years ago. So, you're pretty much out of luck.
Have you looked for a Mac version of the app?
I have a small software application that I want to run on an older mac with os x 10.2.8 (G3 400mHz 320mb). How can I run windows xp or 98 on this mac? I'm looking for a free installation if possible. Any help will be appreciated