You don't give anything like enough detail on your problem rather than your
dealings with Dell to be able to help you much.
I would caution you that moving from Vista to XP is often a major chore if
your are not a bit of a techie. All the drivers for you machine will have to
be located and have to be in versions for XP, or many parts of the system
will not work after the conversion. It's possible the disk they sent you
contains them, but often it is just an XP install disk, and the drivers
won't be there. Be sure you find about that if before you attempt to install
XP.
Lots of folks seem to be under the delusion that Macs are somehow free of
all the problems of PC's, but they ARE PC's and their forums clearly
indicate that there are a raft of problems with them as well. From where I
sit, about all you would be doing is spending more money and probably
getting a new set of problems.
Give it some thought. On the one error you describe, it is usually a
conflict with the Antivirus program on the machine that causes it. Shut off
the Email scanning at least in the Antivirus program. There are hundreds of
posts here having that problem, and most of them remove Norton or the other
installed antivirus systems and install compatible ones. If you want advice,
I'm sure someone will jump in here with some for you.
Take a deep breath, relax, and think about these things. Then do what you
think is right for you.