Fan noise and crashing

Downloading video/audio files should not cause cpu to heat up that much, unless you are running tight on HDD space.  Maybe you are running tight on ram.  If you hit control-alt-del, look at process and performance tabs and you  can find out what process is using up your cpu and memory.  Also, you can install i8kfangui (link under johnallg's signiture block) to monitor the temperature and speed of the fan.  Once you have all these info, you may be able to conclude what is going on.  Otherwise, post these data here and let some experts help you.  Having laptop on for 18hrs is not a problem.  I seldom turn off mine (it falls into standby mode if I am not on it).

Also get a can of compressed air from a computer or office supply store and blow out the fans while running.  If that quiets them then see if that solves the lockups.  If not, get them repaired under warranty or if out then by buying them from Dell Spare Parts - the numbers are below in my sig as is I8kfangui's link, which you should get anyway.