altering config files for repaints

Hi,I`m new to this, so bear with me. Several times I have wanted to repaint my models. Often I have to add something to the aircrafts config file. I open the file, carefully add the lines of info, and save the changes. I then try to open FS2004...I get the opening page..then that is as far as it gets. I have to go ctrl..alt..del to get back to Windows..and task manager says FS2004 is not responding. To get FS running I have to completely reload the aircraft into FS. Just deleting what I`ve added is no good. It`s happened so often, that I must be going wrong somewhere..can someone possibly give me any possible hints as to where I`m going wrong? Stuart.