Problem with client computer asking for credentials to install program

I'm not sure if this is the right place to ask this question. If it is not, then please let me know or move the thread to the right place.

With that aside, here is what I have as of now:
I have a newly installed server running MS Server 2008 R2 (64bit) with Active Directory and DNS (domain is myhome.local). On a different computer, I have three different VMs running Windows 7 Pro (64bit) and all three have been configured to join the domain myhome.local. I have created 3 users (user1, user2 and user3) that are members of domain users only. Which I believe means that those 3 users don't have any kind of administrative rights whether on the server or any workstation.

Here is the problem:
When I log into one of the VMs running Win 7 pro as one of the 3 users I created, say user 1, and I try to install a program like mozilla firefox I get prompt for credentials to install firefox. If I leave the screen as such without typing anything, the window asking for credentials disappears and I am able to continue installing firefox without any problems. Same thing happens if I log into any of the other 2 VMs I created as any of the other 2 users. What is going on here? Why is the window asking for administrative credentials in order to install the program disappears after 1-5 minutes?

Thank you for your responses in advanced.
Andy

 

What are user1, 2, & 3 on the local machine?

 

Ok, so apparently a domain user can install programs that do not require permission to write to any of the windows directories or write to the registry.
Firefox does not need to write to any of the windows folders or the registry, therefore a domain user can install a program like firefox. Any other program like Adobe Acrobat would require domain admin credentials in order to install it for a domain user.