The built in administrator account has two differences from any other administrator account
- It cannot be deleted
- It is not subject to the UAC restrictions
If you choose to disable the UAC then the differences are just the deletability
If UAC is on (good idea IMO
) the normal administrator is stripped of some of the normal admin powers dependent on the level set for UAC.
As malware runs with the rights of the current user running as admin with UAC on deprives the malware of the power to do damage.
Where as normal best practice was to run two accounts a standard for day to day use and an admin only when required, the UAC enables this to be modified (although the older method is still valid, and essential if UAC is off).
My advice is to switch UAC on, create and use an admin account under UAC control for your day to day running, (it does make things easier due to prompted elevation)
If you come to a rare situation where the normal admin is insufficient (sometimes happens due to special UAC controls on the program files and windows folders), enable the full administrator, perform the actions required then disable the account again.
If you share the machine with others in a family - give the other users a standard account only, this will protect the system files from accidental damage
Hope this helps