Some of the MailWasher files may have been corrupted. To fix this:
1. Make a back up of the MailWasher files. If you go to the Start button, type %APPDATA%\Firetrust\MailWasher in the Search bar, and press <Enter>, the location will be opened in Windows Explorer. Make a copy of these files somewhere safe.
2. Uninstall MailWasher, and when asked if you want to keep the data files say No.
3. Reinstall MailWasher.
If MailWasher now runs and you're happy with it, be happy.
If you had things like filters set up and you don't want to recreate them, then:
4. After closing MailWasher, go to the Start button, type %APPDATA%\Firetrust\MailWasher in the Search bar, and press <Enter>, and copy the files there to another safe location.
5. Then try copying the files from step 1 back into the %APPDATA%\Firetrust\MailWasher location one by one, restarting MailWasher after copying each file. If you copy a file and MailWasher fails to start then this is the corrupt file. You will then need to get a copy of this file from the "clean" copy that you made in set 4. After that you can continue copying the files from step 1, testing each time.
Note that the file names in the %APPDATA%\Firetrust\MailWasher location are pretty self explanatory, so I'd only copy the ones that look like they're going to be useful for the bits that are missing from the basic set up you have after reinstalling MailWasher in step 3.