Here it is:
Symptom: Launching Mathcad yields the error:
Microsoft Visual C++ Runtime Library
Runtime Error!
Program: C:\Program Files\Mathsoft\Mathcad \mathcad.exe
This application has requested the runtime to terminate in an unusual way.
Cause 1: The Mathcad installation is missing the prerequisite called MSXML. You can check this by looking in Add/Remove Programs.
Solution 1: Install the prerequisite by running the file "msxml.msi" located in the "Prereq" folder on the Mathcad CD. Then launch Mathcad again.
Cause 2: The Mathcad local assembly cache is corrupted. NOTE: This is more likely with Mathcad 12 than Mathcad 13.
Solution 2: Go to C:\Documents and Settings\(username)\Local Settings\Application Data\Mathsoft\Mathcad and delete the folder named after your version. It is safe to delete that folder since launching Mathcad will result in the creation of a fresh copy of that folder. Next, launch Mathcad again.
Cause 3: You have Mathcad 12.0 or 12.0a and have Microsoft .NET Framework 2.0 installed. You can check the version number by looking in Add/Remove Programs. Mathcad didn't work with Microsoft .NET Framework 2.0 until Service Release 1 for Mathcad 12.
Solution 3: Install Service Release 1 for Mathcad 12 after downloading it from the Mathcad.com Download Center,
Then launch Mathcad again.
Cause 4: The Mathcad installation is missing the prerequisite called Microsoft .NET Framework. You can check this by looking in Add/Remove Programs.
Solution 4: Install the prerequisite by running the file "dotnetfx.exe" located in the "Prereq" folder on the Mathcad CD. Then launch Mathcad again
My problem was with the MSXML file. XP's add/remove programs was used to remove two MSXML files. Then the msxml.msi file was installed from Prereq folder on the Mathcad CD. This fixed the problem. Afterwards, Windows update changed the MSXML file and I had to go through the process again. Since then, I review the files that Windows want to update and don't update the MSXML file (don't know if that is the best thing to do but it works for me).