Microsoft Corp. is probing reports that attackers are exploiting a vulnerability in several versions of its Word document processing software to hijack computers the software is running on. In an advisory Tuesday, the world's largest software maker said that it was looking into reports that opening a malicious Word document could corrupt a computer's memory in a way that lets an attacker run software on the machine. The attack can only occur if users are lured to open a specially designed document they load onto a computer,receive as an e-mail attachment or download from a website, Microsoft said. "Do not open or save Word files that you receive from untrusted sources or that you receive unexpectedly from trusted sources," Microsoft advised as a workaround while it investigates the reports. Security research group French Security Incident Response Team (FrSIRT), an arm of A.D. Consulting Ltd., rated the problem "critical," the most serious status on its four-level scale.