So you're saying, sometimes when you try to open a website/click a link MS Word installation program comes up?
Does the page open even when this happens?
What options does the installer give? Is it just the normal uninstall, repair & modify?
Does this happen on any particular sites?
Have you scanned for spyware/viruses/whatnot? I know, this is a longshot, but I can't think of any other reason for something this weird to be happening...