Installer: Fix the registry-based menu entries for Windows XP (fixes #95)

The non-Background entries require the use of "%1" instead of "%v" on
Windows XP. The Background entries require "%v", but these are not
available under Windows XP anyway. This current solution was also tested
on Windows 8 and it works fine there, too.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>