The problem you are having is that the installer is trying to find the source for this. if it's not avalable, you will have to uninstall. There are several options to you on this as well. If you can find the sourc CD, not just the file, cause there are a lot of files that will be needed, you can save you self some headaches, by doing a custom install of office and removing the unneeded applications, i.e. Frontpage.
Also if you have the space on your machine and you find your source media, place it in a folder somewhere and install it from there. You can acutally do a repair since it's MSI and that will change the source location of the installer.