how to add a new dictionary to word 2003 windows xp

It depends on what you mean by "dictionary."

1. If you want proofing tools for a language that is not supported by your edition of Office, then you must purchase the Microsoft Office 2003 Proofing Tools add-in. This is no longer available directly from Microsoft or from online retailers such as Amazon, but you might be able to locate a copy on eBay or another auction site.
2. If you want to create a new user dictionary, then go to Tools | Options | Spelling & Grammar and click on Custom Dictionaries... Click New..., enter a name for your new dictionary, and click Save. Your new dictionary will then be in the list and you can select it and click Modify... to add words. You must check the box for it if you want Word to use its content, and you must make it the default user dictionary if you want words to be added to it when you right-click on a "misspelled" word and choose Add to Dictionary.
3. To add a third-party dictionary that you have purchased, click Add in the Custom Dictionaries dialog (see #2) and browse to the location of the dictionary on your hard drive.