Yes, they are many resources on the web about this problem, but none helped me. And because I encountered this problem twice and did not remember how I solved it, I thought of posting this, at least as a note to self.
The solutions on web suggest that you are not allowed to read/execute some dlls in the oracle client because rights on that folder are not properly configured. Not the case. Real problem: Missing MSVCR71.dll. Problem with Oracle 10g Instant Client on Vista and Windows 7. Discovered this by using ProcMon to see what my application is searching for in the oracle client folder. The same way I discovered I was missing the Network\admin folder where tnsnames.ora files should be located.