Trying to figure out installed versions/service packs of various things on your NT4 / 2000 / XP System?

I use:

• PSInfo from SysInternals (freeware)
  PsInfo is a command-line tool that gathers key information about the local or remote Windows NT/2000 system, including the type of installation, kernel build, registered organization and owner, number of processors and their type, amount of physical memory, the install date of the system, and if its a trial version, the expiration date.
• MDAC Component Checker
  Component Checker is a customizable tool that performs the following tasks - Identifies the current MDAC installation on a computer, Creates a series of reports about the files identified for the current MDAC installation, Removes the current MDAC installation after listing .dll conflicts and identifying programs that reference a given .dll.
• Shavlik's HFNetChkLT for patch management and yummy security goodness
• The MDAC Installation FAQ with version numbers and a compability matrix for all the Windows OSs (95, 98, ME, 2000, NT4, XP) and the MDACs (2.0, 2.1, 2.5, 2.6, 2.7)

Also, for finding the version of the the MDAC manually via regedit (from Network Simplicity)

Summary:
How to determine which version of MDAC is installed on a Windows machine?
Explanation:
Run regedit and search for the following key:

HKEY_LOCAL_MACHINE
      \Software 
          \Microsoft 
               \DataAccess 
                    \Version