Beware of Xupiter, the Browser Tool from Hell
Some evil programmers whose website resides on Hungary have created a particularly nasty little download that installs itself without your permission and hijacks your browser, periodically changing your start page to the Xupiter.com homepage. Surfers who've been unfortunate enough to have downloaded Xupiter say it's incredibly hard to get rid of. Though it does have an "uninstall" program, it either doesn't work or makes things even worse. If you happen to find yourself at it's mercy, you may have to resort to using something like Spybot Search and Destroy or, for more advanced users, editing your registry.
Having personally dealt with a couple similiar programs that took over my browser settings, I can assure you that nothing will make you madder than connecting to the net and finding that not only has your start page changed (which would be aggravating enough in and of itself) but you can't change it back, no matter how hard you try.
I had one awhile back - EzyCyberSearch - if I recall correctly, that was nearly as frustrating as this and my friend, Phillip, is currently dealing with another, which offers no uninstall or information on it's homepage and even changes your address bar search settings so your browser is directed to their own, utterly useless search. A search for, say, National Geographic won't turn up their website in the results, only links to speciously-related items you can buy on various e-stores. This is the internet at it's worst. I say track these scum down and hang them alongside the spammers, the bot-makers and all the rest of the slimy, sneaky opportunists on the internet and let's be quick about it, okay?