By default Windows 7 asks the user for password each time he tries to add new printer or reinstall one which is already installed with new driver. This behaviour is caused by User Account Control (UAC) which wasn’t available in previous Windows versions, that’s why this problem didn’t occur in the past.
Most often you’ll want to let the user to decide which printer he wants to use, so he should be able to install them without any password prompt unless you want to take full control over all the printers available in your environment together with the responsibility to manage them on each user workstation. This approach will be very time consuming and it can be more easily acquired by printer Access List (ACL).
So, to solve the problem with UAC firing on each printer install you just need to modify following registry key using regedit.exe or some other registry editor:

HKLM\Software\Policies\Microsoft\Windows NT\Printers\PointAndPrint

By adding/modifying following values:

Restricted=dword:0
TrustedServers=dword:0
InForest=dword:0

Remember that after the modification you should restart explorer.exe or whole workstation to reload these settings.