Running as Admin is the type of user account you use. An Administrator account has pretty much global permissions over the computer. A limited user account has much reduced permissions, and generally can not install nor uninstall nor update most software that affect more than that account.
Go to Control Pnael>User Accounts and look at the account you use. (Your logon.) It will say whether it is an admin account or not.
The options on the site I linked are generally intended to be sequential. If the first one doesn't solve the problem, try the second. If that doesn't help (and you still want it fixed badly enough) try the third, if you're up for it.
Think about what is different between the two machines. Do they both have the same settings, firewall, and antivirus? Is anything running on the primary that isn't present on the backup?