I design my custom guitars in 3D Studio Max 5.0, which is a good program for the task, especially because it is comatible to Autocad and has a very advanced material editor which allowed me to do a realistic quilted maple top with a shiny sunburst finish. It is a very complicated program aiming at the pro though. It is usually used for making rendered films and animations for Videogames and TV.
Dont expect to simply draw a guitar....You really have to learn the program first, but it allows to draw virtually EVERYTHING, as can be seen in many commercial films and games. Another downside is it's high price....but if you only need it for private use and dont want to sell/show the pictures to customers you could get a pirated version....
HTH,
Marcel Knapp!