See what PeterT said above -- he got to it before I did.
Also:
Sometimes newer versions of Calibre will work okay with Windows 7, though it heavily depends on your setup. You can try downloading + unpacking a
portable version and see if it runs.
Note that you
do not want to open your existing library with it unless you want to upgrade permanently. There are database incompatbilities introduced in 5.x that will result in errors on older version.