Hackers on Tuesday hijacked the Web site CheckFree.com, one of the largest online bill payment companies, redirecting an unknown number of visitors to a Web address that tried to install malicious software on visitors' computers, the company said today. The attack, first reported by The Register, a security news Web site, began in the early morning hours of Dec. 2, when Checkfree's home page and the customer login page were redirected to a server in the Ukraine. CheckFree spokeswoman Melanie Tolley said users who visited the sites during the attack would have been redirected to a blank page that tried...