IMPORTANT: Microsoft is replacing Windows Live Messenger with Skype. All your contacts will be moved automatically. Take a look at The Complete Guide to Moving from Messenger to Skype for more information, alternatives and tips.
MSN Messenger is a great app but it does tend to leave all kinds of files and folders on your system even after you've uninstalled it.
This can prove a real problem when you want to install another version of MSN Messenger. If you've ever received a message such as 'Another version of the product is already installed' or 'Your computer has a newer version of MSN Messenger than the one you are trying to install' then you'll know what I mean.
ZapMessenger is an extremely simple application which removes any trace of MSN Messenger from your system after installation. It's designed as a single-step removal solution for several versions of Messenger including MSN Messenger, Windows Live Messenger, and Windows Messenger 5.x.
When you run ZapMessenger, the first thing it will attempt is to uninstall Messenger normally. If that fails, it will force-remove Messenger from Windows allowing you to proceed with other installations. There's not much to keep you updated on what it's doing other than a success or fail message at the end of the process. There are no customization options either and bear in mind that it may not work with applications such as MessengerPlus!
ZapMessenger could help you if you're having problems installing an older or newer version of messenger than the one you are currently using.