The newest ACR versions only work with Windows 7 and higher, so that could be why nothing is updating.
Adobe has a help page with instructions for older-OS systems that tell how to manually install ACR 8.3, which is the newest version that works on Vista.
http://helpx.adobe.com/x-productkb/multi/camera-raw-84-support-policy.html
This help page was made because the first ACR 8.4 installer wasn’t smart enough to look at the OS and went ahead an installed ACR 8.4 even if that version wouldn’t run on the OS, so that is the scenario the instructions apply to most directly, but they should also work to upgrade an older ACR 7.x to ACR 8.3. You want to pay attention to whether you’re installing the 64-bit plug-in or the 32-bit plug-in because you only have 32-bit Windows.
