I have a Windows Vista laptop and an XP desktop and this works on both of those platforms to increase the help file font size. I'm not sure if it is in other versions of windows.

After opening the BIAB help file, in the top menu click Options, Internet Options..., Accessibility, and then check the checkbox "Ignore font sizes specified on webpages" and then click OK, OK. The font in the help file should increase. The font size is controlled by the settings you have in Internet Explorer, whether you use it or not so if you set the font size in IE to largest to get the biggest font in the help file. (You have to quit IE and reopen the help file for the font change to take effect.

Hope that helps.