Washingtonpost.com and Security Fix readers may have noticed that our site was a bit slow and occasionally unreachable today. Turns out, the site has been under attack by about 60,000 compromised PCs around the globe for several hours now. We weren’t the only site reportedly picked on, though. According to several security researchers who asked to remain anonymous because they are still helping to investigate the assault, the same attackers targeted Web sites for the White House, the Department of Homeland Security, the Department of Defense and the Federal Aviation Administration, with varying success. The culprit is a piece of malicious software that orders infected PCs to visit the Web sites on its hit list over and over again, all in an apparent bid to render the targets unreachable to legitimate visitors. Joe Stewart, director of malware research at Atlanta-based SecureWorks, said he examined the attack software and found that