read the threads on people who get modified content error when trying to play NS. it seems to be the same bug & there have been 2 pretty solid fixes. one is using the incremental patch 1.40 to 1.50. there is something in the full 1.50 patch that causes this problem. the other is a registry edit because your pc thinks you are running an outdated version. i had the same exact prob & went through hell only to find out that the full 1.50 was causing that prob. so if you are using it, trying installing 1.40, then the incremental patch...should work. best of luck to you.