What is the difference between my admin account and the "Administrator"?

I created an admin profile (Admin1) when I installed Windows 7 Pro (not talking about the Administrator profile though). Admin1 is in group of Administrators but there are still things I can't do like modify certain files or install rights unless I log in as Administrator.

What's the difference between my Admin1 (which is in Administrators group) and Administrator?

If you didn't turn of User Account Control, by default in Windows 7 the Built-in Administrator account doesn't require privilege elevation, while users you create and put in Administrators group do.

That is if you want to do anything that requires Administrators rights with the account you created and placed in Administrators group you will have to either confirm the privilege elevation when requested by Consent UI or do run as Administrator by right clicking the application and selecting the option.

In case of MSI packets if you don't get the Consent UI, you have to start command prompt as Administrator (run as) and start the MSI from that command prompt session.

In case of system files that means the following:

If you lets say want to modify hosts file (or some other file that requires Administrator rights) with the account you created and placed in Administrators you will have to right click some text editor (like Notepad) and select run as Administrator and then open the hosts file from the File menu by selecting Open.