Sometimes you may not be able to uninstall a piece of software becuase it complains that it can't find the original installer. Becuase of this you then can't install an updated version because it's complaining that you already have it installed.

Microsoft have this tool http://support.microsoft.com/kb/290301/ which can help in these situations.

Be carefull though -this DOESN'T uninstall the software, it just makes Windows 'forget' that it is installed - all the icons and software is still there. This therefore could cause real problems with installing new versions, or even make windows unstable. So basically, you really don't want to have to use this unless you have to!

That said, I've just had to use it here to fix a PC that just wouldn't install a new version of a program and it got me out of a whole!