%SYSTEMROOT% == c:\windows (or the root windows directory)

%SYSTEMDRIVE% == the drive windows is installed to (eg, C:\)

How can I get the system drive letter?

There's an environment variable called SystemDrive which is set to the system drive (surprisingly enough). The getenv() call is how you can get to it.

char *sysDrive = getenv ("SystemDrive");
if (sysDrive == NULL) {
    // vote me down.
} else {
    // vote me up and use it.
}

if(DWORD len = ::ExpandEnvironmentStrings(input, 0, 0)) {
if(len > 0) {
TCHAR *cp = new TCHAR[len];

if(cp) {
if(::ExpandEnvironmentStrings(input, cp, len) != 0) {

result = string(encodings::utf8, cp);
}
delete[] cp;
}
}
}