Does this only occur with 3rd party applications?
It could be permissions related, but you'd need to identify which applications this behavior is happening with. In Leopard, library files are sorted in order of ~/Library, /Library, then lastly /System/Library. So this means that if the particular application is only installing supportive files in the ~/Library (that's the User Library folder) then other users won't have access to the updates, hence your a plausible cause of your error.
I'd first repair permissions using Disk Utility and see if that remedies the problem. If that doesn't work then track down if the particular application puts supportive files in just your ~/Library folder or both ~/Library and /Library