Researchers at Symantec said late Tuesday they've spotted a Web site that tries to exploit computers lacking one of the recently issued patches for versions of Microsoft's Excel spreadsheet program.The vulnerability involves a malicious Excel file that when opened can allow a hacker to execute other code on a PC. In its advisory, Symantec said one Web server is hosting the malicious file, which it calls "Trojan.Mdropper.AA." The Trojan Horse spreadsheet document can download other harmful files to a computer.Users could become infected if they open the malicious Excel file sent to them as an e-mail attachment. Also, they could be redirected to the Web site hosting the file by an iframe embedded in a page on a compromised Web site, the vendor said.To protect computers, users are advised to apply MS08-014, the batch of patches Microsoft released earlier in the month that fixes several flaws in Excel 2000, 2002, 2003 and 2007 for Windows. Administrators should also block their users from accessing "intop.info," the site hosting the file.On March 11, Microsoft released four critical updates that fixed 12 vulnerabilities for its Excel, Outlook, Office 2000 and Office's Web components.