Why is "Save" grayed out on MS-Word 2003?

Q: I'm able to open a document in MS-Word 2003, but not able to make changes or save it. Why is "Save" grayed out on MS-Word 2003?

A: After some digging, it appeared that a virus had infected the user's system and de-activated Office 2003. After retrieving the registration number and re-activating MS-Office, everything should work fine as expected.