A Mathcad directory called the local assembly cache may have become corrupted.

Therefore, to resolve this issue:

1. With Mathcad closed, locate the following directory, C:\Documents and Settings\%username%\Local Settings\Application Data\Mathsoft\Mathcad

2. Delete the subdirectory called "Mathcad." That directory is the local assembly cache and is recreated with clean files when you re-launch Mathcad.

Please note that "Local Settings" is a hidden directory, so you may need to make your hidden folders visible using Tools/Folder Options/View.