You asked between 2000 & 2003, not XP & 2003, so here are SOME differences:
File/New gives the task pane
Task pane has dropdown for New Workbook, Clipboard, Search, Insert
ClipArt
File/Open dialog box has "Open and Repair." option
Ask A Question commandbar
Function Tooltips when typing a function
Colored Sheet Tabs
Insert/Symbol
Worksheet Protection features
Tools/Protection/Allow Users to Edit Ranges
Ctrl/C twice fast brings up Clipboard; or Edit/Clipboard
Clipboard taskpane Options button
Paste options smart tag
New options in Paste Special
Copy/Paste need not be same size. If not multiple of original, gets
original shape.
Press a cell w/Hyperlink (not click) changes to a plus pointer w/o
invoking link
Fixing formulas tag -- when formula omits adjacent cell.
Find within sheet/workbook
Find Formatting
move tbbs w/alt/drag
Format Cells contains a Locale setting on number tab when "Special" is
selected
Distribute text in cells
Merge button is a toggle
Draw borders
Tools/Error checking
Tools/Formula Auditing/Evaluate formula
Tools/Formula Auditing/Show Watch Window
Drawing objects have builtin rotation option
Camera tool takes bad pictures(!)
Graphics in Headers & Footers
Full file path in header
Print won't print blank pages
Auto-sum includes other than sum
Open/Save files in XML
Edit links contains a Startup Prompt button
Pivot tables now contain a field list view
Pivot table tb contains PivotTable/Order/Move Left.
Autosave builtin to save tab
File/Open, Save is resizable
Page setup has feature to show how errors are printed
deleting a never-used sheet doesn't alert
New tabs in Tools/Options
Narrow column tooltip -- hover over ### to see contents