I'm not really sure what the world's worst installer actually is but I've got a pretty strong candidate for it. Adobe's Creative Suite 3 Web Premium (Jesus H Christ what a mouthful) takes about a billion years to install and then, even worse, when you want to get rid of it its not very clear how to. There's no remove program link on the start menu, in add/remove programs it isn't listed under Adobe, or Creative Suite 3 or Web Premium. In fact its semi hidden under the words Add or Remove (not that intuitive).
Then when you do start the uninstall process, it asks you to close down practically everything running. It ordered me to shut Firefox, Excel, Windows Live Messenger, Flash 8 (which isn't even part of the creative suite, its the previous version) and then finally, when it was happy all I had open was Notepad, it took ten minutes to uninstall. So I sit and stare at a progress bar, willing it to move forwards pixel by pixel, as it has reduced my machine to having no programs running at all so I cannot do anything.
For gods sake, who coded this mess. All it has to do is delete a bunch of files, their folders and remove some registry entries. The only way they could have made this any worse - yet still work - would be if it forced me to reboot the machine afterwards. Well, it tried to make me by "recommending" it but at least it didn't force it to happen.