There are some real gems around when it comes to open source software. One of the best is Open Office, a completely free alternative to Microsoft Office.
Imagine my shock when all of a sudden Open Office started randomly crashing. Reinstalling didn't work - it wasn't possible to get past the registration screen. All sorts of dark thoughts ensued including the unthinkable - buying a copy of Microsoft Office! But when it comes to software, there is always a reason why things don't work. After a bit of detective work, I traced the problems back to installing Microsoft's Internet Explorer 9, which I don't use for browsing but keep for checking websites when developing.
Unbelievable as it may seem, IE9 somehow manages to prevent Open Office from working. I'm sure more conspiracy minded people would put this down as no coincidence, but I prefer to assume incompetence until proven otherwise! So, if your Open Office stops working, don't uninstall it. Just un- install IE9 and you will probably be ok.