After installing ChemDraw, I can no longer open Microsoft Help files using the CHM extension because ChemDraw attempts to open them and can't.

After installing ChemDraw, I can no longer open Microsoft Help files using the CHM extension because ChemDraw attempts to open them and can't.

The CHM format is the original ChemDraw 3.x format.  We have been using it for years.  Unfortunately, Microsoft decided to use that extension for Help files even though we were already using it (because there is no registration system for extensions - any software developer can use any extension). 

To reset the CHM extension to Microsoft Help, you will need to do the following:

1. Open the Windows Explorer.
2. Go to the View Menu and choose Options (Folder Options for Windows 98).
3. Click on the File Types tab.
4. Scroll down the list of Reegistered File Types until you find CS ChemDraw 3.5 Drawing.
5. Click on it and click on the Remove button.
6. Click the New Type button.
7. Set the Description to Microsoft Help.
8. Set the Associated Extension to CHM
9. In the Actions section click on the New button.
10. In the Action field, type Open.
11. Click on the Browse button, and browse to the HH.exe file in the Windows (or WinNT) folder.
12. Click Ok, click Ok, and click Ok.

However, ChemDraw 5.0 and lower will reset the CHM extension to ChemDraw the next time you launch ChemDraw. ChemDraw 6.0 and above will not reset the CHM extension to ChemDraw, so unless you don't mind resetting the CHM file type, you will want to upgrade to the latest version of ChemDraw.

To open ChemDraw CHM files in ChemDraw, you will need to launch ChemDraw, and open the file from within ChemDraw. If you are using ChemDraw 4.x or above, you can rename all .CHM files to .CDX, which is now the current default ChemDraw format. You can use the DOS RENAME command to rename multiple files at once.