There are a few things you can do to make your system run a bit better.
Disable what you don't need (hardware)
Go into your bios and turn off the things you don't need. If you are not using your parallel or serial ports, turn them off! If you aren't using your integrated modem, turn it off!
However, I would suggest keeping a record of what you do turn off. That way you can have a first step in trouble shooting why a device isn't working.
Remove windows components you don't need.
Chances are you probably don't need MSN explorer.
in windows XP open add remove programs control panel and click on add/remove windows components. Then go through and remove what you don't need.
If you aren't using MSN explorer or Outlook Express, it is just taking up space and providing potentially security issues.
Update your dell firmware
http://support.dell.com has updates for just all about of their models. Most of the updates help performance, compatability, or reliability. Just make sure you don't lose power to the system during the update, and back up your data first.