The error you got means that the dependencies checking script has an error. Check the package checksum to ensure it is not corrupted. If the checksum is correct, then try installing the update on your current drive from another snow leopard install.

The checksum for your 10.6.5 combo update should be:

SHA-=add336a1af1c3914887d2217fbbc98b18e6fb57c

To check it, please visit the link below:

http://www.bresink.com/osx/sha1.html