Best thing is to remove all instances of the software from the registry. To do this you need to run regedit - click on 'My computer' and then do a search for the title of your software. Be very careful though. Most registry entries for a particular piece of software are stored in a location or hive. Any obsure entries should be ignored. If in doubt - do not delete these registry entries.
As far as unistall software is concerned you may like to try using Norton's 'Uninstall'. However, the drawback with this type of software is that your machine will invariably slow down as it has to scan the registry everytime you install a piece of software.
Hope this helps.
IT Guru