I've upgraded from Word 2000 to Word 2003, but Word 2003 runs so slowly that I wish I hadn't bothered.
Word 2003 is much more demanding than Word 2000 and typically runs more slowly. Choose Tools » Options and try the following suggestions for improving performance:
On the Spelling & Grammar tab, uncheck the "Check grammar as you type" box. If you can do without on-the-fly spellchecking, uncheck the "Check spelling as you type" box as well.
On the View tab, uncheck whichever of the following you can dispense with: the Highlight box, the ScreenTips box, the Smart Tags box, and the Animated Text box. You may want to leave the ScreenTips box checked if you use revision marks in your documents.
On the General tab, uncheck the "Provide feedback with animation" box.
On the Save tab, uncheck the "Embed smart tags" box. If you save your documents diligently, uncheck the "Save AutoRecover info every" box as well.
On the Edit tab, make sure the "Mark formatting inconsistencies" and "Auto-Keyboard switching" boxes are unchecked. Depending on your editing preferences, also consider switching off the "Use smart cursoring," "Use smart paragraph selection," "When selecting, automatically select entire word," "Keep track of formatting," "Show Paste Options button," "Smart cut and paste," and "Enable click and type" options. If you're not clear what these options do.
Word may also be suffering from a surfeit of fonts. If the Font drop-down list or Font dialog box includes fonts you never use, consider removing some. Choose Start » Control Panel » Appearance and Themes » Fonts to open the Fonts folder, select the fonts you don't need, and drag them to another folder for temporary storage. (You can delete the surplus fonts if you prefer, but it's usually better to keep them in case you need them later.)
Warning: Don't remove any of the following fonts that Windows requires for its interface: Arial, Microsoft Sans Serif, Tahoma, Marlett, Times New Roman, Courier New, Symbol, and Wingdings. Windows protects Marlett and doesn't display it in the Fonts folder.