I have not had any experience in the "low volume on mic input."
if Greg's suggestions above don't help, (which I'm afraid they might not), then I'm going to have to agree with him that the program might not work correctly with the newer OS - after all, it wasn't designed to work with it. win7 wasn't even in development when PA9 was released.
check your settings on the realtek in windows, check what greg has given you and if neither of those work, then buying a cheap soundcard might be your only option to get it to work right - or possibly (and I don't say this is definitive) upgrading to MC6 might work.
upgrading to MC6: $50 download
new soundcard: $100 is about the cheapest you can get with a reliable mic pre and reliable drivers for win7.
I know you've already said more than once you're not interested in buying a soundcard, but if you change your mind, there are some recommendations on my website. I would not recommend something like the cheap behringer UCA in your case because there's no mic pres on it and you'll have the same low volume problem.