Cannot use MS Office anymore

A couple months ago, I went to open a Word document and came up with a small window with the heading “WINDOWS INSTALLER” and inside the window it said “Preparing to install…”. I had used Word just the day before with no problems and now there wasthis problem the very next day. I reviewed everything I had done the previous day and there were no changes made, no software installations done and no settings altered. When I cancel out of this window, another message pops up saying there is a problem with the “registration” of the Windows Installer. This is a Win2k machine. Can I re-install the Windows installer? How do I do this? I keep Symantec Anti-virus up to date and I have had no viruses.