If you are one of the persons who are experimenting problems saving your data and also getting the achievements during the game. There are 2 ways to fix the problem.
1) When you install the game change the location of your game to root folder or some other folder that UAC can't make any problems
2)go to C:/Program Files(x86)/HeXentic Games/Infernal Underground/bin64/ and click properties in "sauerbraten.exe" and change the compatibility settings in "Always run as admin" and save. (If you use the 32 bit version of the game do the same process in the bin folder)
After that just use the .bat file in the Infernal Underground's root folder to play the game