The home directory isn't really a cross-platform concept. Your suggestion of the root of the profile directory (%USERPROFILE%) is a fair analogy, but depending what you want to do once you have the directory, you might want one of the Application Data directories, or the user's My Documents. On UNIX, you might create a hidden ".myapp" in the home directory to keep your files in, but that's not right on Windows.
Your best bet is to write specific code for each platform, to get at the directory you want in each case. Depending how correct you want to be, it might be enough to use env vars: HOME on UNIX, USERPROFILE or APPDATA (depending what you need) on Windows.
On UNIX at least (any Windows folks care to comment?), it's usually good practice to use the HOME environment variable if it's set, even if it disagrees with the directory specific in the password file. Then, on the odd occasion when users want all apps to read their data from a different directory, it will still work.