Unlock causes application to hang

I have a newly bought iMac with OS X 10.6.8

When I try to unlock to make changes (eg. changing accounts, installing SW etc) the application hangs with the beachball spinning and I have to manually terminate the application (which is in state "not responding")

Any ideas what to do? Otherwise everything functions ok.

Can you tell us how you're installing the applications? I'm not sure what you mean by "unlock to make changes". If you're the administrator you should be able to install an application without changing accounts.

Installing an application on a Mac is straight forward. Normally an application either comes with an installer, or it may be just a .DMG file that you've downloaded. Can you be more specific as to exactly what it is you're doing and which applications are you trying to install?

I really mean "unable to unlock". When I am in the account settings menu as administrator and I want to make changes, I click the lock and the menu hangs with the beachball turning and I have to abort it manually.

I guess it is the same problem when I try to install applications (eg. virtualbox, monodevelop) from .dmg: there should appear an administrator password menu but the install hangs.