Here is a compilation of solutions to this error, Hopefully one works for your situation
1. Anti-Virus programs (Namely AVAST in this case) and Firewalls can alter your Crossfire program when you install it or as you run the process - If you suspect your Anti Virus program or Firewall may be doing this - Disable them when reinstalling and add Crossfire to their exceptions list. For some programs such as AVAST this is simply not enough - You should consider replacing your Anti Virus program in this case.
From Z8games " Opening up AVAST "Standard Shield" then click on "Customize" when in "Advanced section" You must add the path of your crossfire.exe (C:\Program Files\Z8Games\CrossFire\crossfire.exe)"
2. If you receive any "File is changed" Error I highly recommend this post by the GM of Z8games;
"- Uninstall the game completely
- Disable your antivirus' realtime protection
- Redownload the game
- Install the game
- Add the entire crossfire folder to your antivirus exceptions
- Re-enable realtime protection"
Z8Games - View Single Post - [SOLVED] Common Solutions to Cross Fire Errors
3. A program such as Xfire, FRAPS, Skype, Or a variety of other things may be conflicting with Crossfire when you run it. You may want to close all unnecessary programs by manually disabling them when playing or by using a program such as SmartClose;
SmartClose download and reviews from SnapFiles
4.. If nothing provided here or in any other suggestions works - I highly recommend you contact Z8games support with a concise explanation of the error;
- Give them the error in text format
- Give them the error in a Screenshot
- Explain your System's Specifications
Good luck!