In MDAC, 'rollbacks' mean going backward to an earlier version of MDAC (for example, going from MDAC 2.6 SP1 to MDAC 2.5 RTM). Rollbacks are highly discouraged If at all possible, do not roll back.

In a successful rollback, all the files are not actually rolled back (this can cause problems). Depending on the operating system installed, a rollback may be impossible

• On a Windows NT 4.0 or Windows 98 operating system, rollback operations are done with the 'Reconfigure' utility that is included with the Component Checker.
• On Windows Millennium Edition or later, rollbacks are done by running

  dasetup.exe /u

• On Windows Millennium Edition or later, there must be a 'SavedRollbackVersion' available to roll back

If the operating system is running a service pack, and a previous configuration of the operating system was saved when the operating system was upgraded, the operating system can be rolled back to the last known configuration.

• Re-install the operating system
• Re-image the computer if an image is available