Changing Product Key Win 98 re-install

I have a user who has a computer with Win98SE already on it but would like to change the Product Key/ID to a license key that they already own with out having to re-install other programs. When they try to do it, they reach the point in the installwhere it askes for the Code and the code is rejected and they cannot go any farther. Microsoft site said to disable any virus scanning software. Made no difference. Any ideas?