one day i'll put my mind to it and write spyware for mozilla browsers... that'll silence all you firefox fan boys...
before anyone says anything about how much more robust or infallible firefox is...
there is a bugs tree, so there are vulenarabilities...
I feel the best suggestion is actually to increase the security settings inside of IE, and implement either the advanced version of adaware, (which you'll have to buy) or spybot (which is free) use the imunize type functions to block known bad content and spyware, and use the function called tea timer (in spybot) to monitor registrt changes to make sure that nothing is automatically installed or anything whilst you browse...
perhaps it's a good time to get a personal firewall as well.. and most likely an anti virus package. (which will usually detect the trojan type software that installs this stuff without you knowing.)
last but not least, in addition to running spybot adaware you could also run Hijack this, the log file is a little hard to understand, so if you get stuck with the results then post them back here...
a thing that you could do, is just format everything and reinstall windows, just to make sure you get rid of all the rubbish that was left on the machine by the old user.