INSTALLATION:
So, you installed Windows XP on the new 120GB HD?
Fresh install from Windows XP installation CD?

Did you instal all drivers from the Dell driver CD (any question marks in Device Manager)? That is an older Dell model, so check Dell.com to make sure you have the latest drivers installed for all components.

Did you run Windows Update after XP installation?

NETWORK:
Did you run Network Setup Wizard on the new XP installation?
What firewall is running on the new XP installation?