I think the route cause is McAfee. A few of the users on this forum have had issues with McAfee.

One other thing to check would be to do the following;

Press the Windows key + R
Type regedit click OK followed by Yes on the User Account Control window
Locate HKEY_CURRENT_USER\Software\YTKE
On the right side look for YMPath near the bottom
Right click YMPath, Click Modify, Under Value Data enter the path C:\Program Files\Yahoo!\Messenger\YahooMessenger.exe and click OK. (For Windows 64 bit it would be C:\Program Files (x86)\Yahoo!\Messenger\YahooMessenger.exe )

Then try YTK Enhanced again and see if Yahoo! Messenger loads.