From MDLWiki

[edit] Problems with Norton Anti-Virus

If you have a problem with Norton, Windows Live Messenger 8.5 and Messenger Discovery Live, follow the instructions below:

1. Uninstall Messenger Discovery Live by following these instructions here: Uninstall.
2. Disable "Auto-Protect" on Norton.
3. Reinstall Messenger Discovery Live from the site.
4. When setup of Messenger Discovery Live is complete, go to Norton "Options" and under "Exclusions" for both "Auto-Protect" and "Manual" scan, browse for the Messenger Discovery Live file and add it as an exclusion.
5. In Norton "Options" under "Internet Worm Protection" go to "Program Control" and add the Messenger Discovery Live program and loader to the program list by browsing for the file. Choose "Allow" next to both programs.
6. Enable "Auto-Protect" again.
7. Open Windows Live Messenger and Messenger Discovery Live should open with it.
8. Sometimes Norton may still do a quickscan and detect Messenger Discovery Live removing it and sending it to quarantine.
   Go to Norton "Quarantine" and you will find Messenger Discovery Live removed.
9. Click "More details" and then click "Restore".
10. You will be asked to confirm to restore. There should be a checkbox that says "Do not detect in future scans"; make sure it is checked, then confirm the restore.
11. Sometimes the file "winmm.dll" may be missing from the "Windows Live" file where Windows Live Messenger is. Search other Messenger folders like "Messenger" or "MSN Messenger" to find it and copy and paste it into the "Windows Live" file. To view the file "winmm.dll" you may need to view hidden files. Click "Tools" > "Folder Options" > "View" and select "show hidden files and folders" and click "Ok".
12. Restart your computer to be sure all works correct.

Everything should work after this procedure.
Questions and problems you can ask on the forum.