My husband's computer had a virus along these lines a year or two ago, hijacking Google results, and that thing was tough to get rid of. Not a single malware scanner found it. I simply noticed because he complained his computer wasn't working right, and the usual scanner wasn't fixing it for him. Neither was any other scanner I tried, and I tried a bunch. Not one so much as detected it, but the changed search results showed that something was going on. I had to do a reinstall on his computer too.
Found it his computer probably got infected because he kept going back to a site his scanner warned him was infected, and he'd ignored the warning. *headdesk* I think he knows better than to make that mistake again, but there are reasons why I don't like him using my computer. You'd think the previous discussions we'd had about malware would be enough... I hope he finally has that lesson down.
Going to have to keep that Sysinternals suite mentioned elsewhere in mind. He's not the only problem child in the family, although I have most relatives pretty well trained now.