This could be a poorly designed macro interfering, or a damaged Office installation. To rule out the first issue, start Word in Safe mode (as a troubleshooting step).
Here's how to start in Safe mode: Exit Word if it is currently running. Hold the Ctrl key as you start Word the way you normally would; when asked about Safe mode, click yes.
If Safe mode works, troubleshoot for add-ins as discussed at
http://support.microsoft.com/kb/921541/en-us.
If it doesn't work, try repairing Office via Help | Detect and Repair. You can also use the installation CD for Office 2003 and repair from the disc, which is usually more effective.