When Vista's service pack version is at level 1 (SP1), this version number is shown at "Control Panel / System". However, on my system this information has disappeared, probably because I'm now using SP2. I'd like to verify this, so I can be certain.
Conclusion: Thanks to Ivo I found the registry settings that store the service pack version number. Because I was (incorrectly) convinced that I had installed a service pack already, and because the "CSDBuildNumber" was 2 on my system, I did some research about the info in the registry. Here are the results:
Fresh Windows Vista image without SP:
- CSDBuildNumber: 2
- CSDVersion is not present
Fresh Windows Vista image with SP1:
- CSDBuildNumber: 1616
- CSDVersion: Service Pack 1
Fresh Windows Vista image with SP2:
- CSDBuildNumber: 1621
- CSDVersion: Service Pack 2
So indeed, my problem was that - contrary to what I believed - I had no service pack installed. Thus, the "Control Panel / System" dialog can be trusted.