Apache module can not be activated in XAMPP control panel

When I started Apache module in XAMPP control panel, it could not run; so I changed the Listen port from 80 to 81 as below in httpd.conf :

#Listen 12.34.56.78:80
Listen 81
ServerName localhost:81

and here is the result again :

09:25:20 ب.ظ  [Apache]  Status change detected: stopped
09:25:20 ب.ظ  [Apache]  Error: Apache shutdown unexpectedly.
09:25:20 ب.ظ  [Apache]  This may be due to a blocked port, missing dependencies, 
09:25:20 ب.ظ  [Apache]  improper privileges, a crash, or a shutdown by another method.
09:25:20 ب.ظ  [Apache]  Press the Logs button to view error logs and check
09:25:20 ب.ظ  [Apache]  the Windows Event Viewer for more clues
09:25:20 ب.ظ  [Apache]  If you need more help, copy and post this
09:25:20 ب.ظ  [Apache]  entire log window on the forums

is there any way to activate Apache ?