Before attempting to play a multiplayer match, it is imperative to have the correct version of the game you wish to play.
The current patch for X-Wing Alliance is the 2.02 patch.
X-Wing Alliance 2.02 Update: HERE
The current patch for X-Wing vs. TIE Fighter is the 1.1.4 patch. When correctly installed, the game version at the bottom right corner of the datapad will be displayed as 1.1.
Note that if you have installed the Balance of Power expansion pack, you do not need to update X-Wing vs. TIE Fighter, as Balance of Power includes the 1.1.4 upgrade.
X-Wing vs. TIE Fighter 1.1.4 Update: HERE
The Emperor’s Hammer’s Science Office creates many custom patches designed primarily for single-player use. It is a good idea to remove any such patches before playing, as some may cause a file conflict between your game and your opponents’, causing the game to crash. It is impossible to gain any advantage by modifying the game files, as the game will simply refuse to play when it detects the difference.
The only exception to this rule is if you are participating in a competition where ship patches are allowed and you have agreed with your opponent(s) to the use of the patch. If all players use the same patch, there will be no mismatch in files and the game will work correctly.