I have had similar problems. First things is older laptops will be slower and the clock rates inside are slower and the software is getting mor demanding on speed.
I would recommend reinstalling XP (I assume it's this) to clean everything out). You should have the DELL discs, if not get them from DELL. Make sure you extract all Keys from the laptop first so the reinstall is legit and you can get all the updates (SP3 etc).
If you don't want to do that get the audio driver from DELL (that's what I did on the same laptop some time ago).
I would not think the laptop can be upgraded to Vista (too slow) but you may consider Ubuntu as an alternative (may not be quicker but just a thought).
Google your problems and you should get answers for a common laptop such as that one.