This is an old issue with the game and randomly happens. The best thing to do, is make sure nothing is running in the back ground - like explorer or anti virus software( just becareful if you switch this off and on broadband).
Also, its worth updating your graphic card drivers - but this won't be the cause. If you are playing from a previous saved game, where it did crash, it will continue at more or less the same point. Bug in the game im afraid to say
you will need to make regular saves. it could also be your firewall causing these problems. To deal with firewall issues, http://www.lucasforums.com/showthread.php?t=163382 that thread will help. It IS Image heavy, so be patient for it all to download