Since you have 11.04 working then it may well be just a bad upgrade to
11.10 - you'll read in here and on any Ubuntu help sites that it's a
bit of 'pot-luck' at times whether an upgrade goes cleanly or not, and
generally the more you update the greater the chance of it all going
horribly wrong :-(

In general, hassle that it is a clean install is always preferred and
that goes for any OS - not just Ubuntu.

I'd really recommend that you do a clean install of 11.10 to verify
whether your problems are due to the new version or not. Ubuntu is
good on old hardware but there are major changes going on and some
older gear is going to struggle unfortunately. The Dimension 2400 is
pretty long in the tooth in computer terms :-(