The top one is the default since that is the original drive that came with the computer.  [URL]  However-- you should not use XP on the Internet since it has been obsolete for a few years now and is not safe. MS has stopped issues any updates or security fixes. The 2350 is 16(!) yrs old now and really ready for the recycling bin.  [URL]
I am having a fight with myself if I should put the reinstall XP disk in the top or bottom disk drive. Help will be nice.
Help with reinstalling Windows XP on Dell Dimension 2350.