Want to cause all sorts of havoc on a PC? Install the Google Toolbar and make sure that the setting that protects Google as your search provider and home page is set (I think it is by default). Then install Yahoo!'s toolbar and make sure the same settings are set on there. Watch the fun ensue.
Seriously, don't do the above. I has a user complain about "some sort of Yahoo error that was causing him to not be able to work." I went over there, and the Yahoo! search protection was trying to protect the Google software from updating the start page, and the Google toolbar was fighting with the Yahoo! toolbar to do the same. A constant stream of balloons and messages were popping up, with one program trying to out-protect the other. Once I uninstalled one, everything worked fine again.
Wonder what would happen if you threw the Ask Toolbar into the mix, as I'm sure it has similar features? I have an old XP system in the office, I'm almost tempted to try this.