Hackers have stepped up attacks on computers running some versions of Windows, a day after Microsoft disclosed a hole related to the mouse cursor. Microsoft Corp. sent out a security advisory Thursday warning customers that a vulnerability in ".ani" files - used to change the cursor into an hourglass while a program works, or into a dancing animal or other animation on specially designed Web sites - was allowing hackers to break into computers and install malicious software. "Overnight we did see the attacks change from limited and targeted attacks to slightly more, but do still categorize it as a limited attack," Mark Miller, director of the software maker's security response group, said Friday. The so-called zero-day attack, a vulnerability that is discovered before Microsoft has a chance to fix the problem, is aimed at PCs running Windows Vista, the new operating system that the company has touted as its most secure. The hole has also been found on Windows 2000 Service Pack 4, Windows XP Service Pack 2 and some versions of Windows Server 2003.