Alexander: Who can you believe about PC infections?

QI'm beginning to have doubts about how safe my computer is.

It began with an odd phone call from the 801 area code. A woman with a very thick foreign accent told me that my computer was infected with malicious software. I asked for her supervisor, who also had a heavy accent, and was told I should download a fix for my computer. I refused and asked her to send further information by mail -- she already had my street address.

Was this a legitimate call from Microsoft?

Then, after reading your column about an FBI investigation that discovered 570,000 computers worldwide had been taken over by malicious software, I went to the FBI-authorized website you suggested. The site said I wasn't infected, but it warned that the test for malicious software might not give the right answer.

"If your ISP is redirecting DNS traffic for its customers you would have reached this site even though you are infected," it said.

My Internet service is through Comcast. Can I believe the FBI test results that say my computer is OK?

JIM GESSELE, Minneapolis

The phone call was a scam. A Web search shows that the number you mentioned has called other computer owners with similar claims. When met with skepticism, the caller usually hangs up.

As a general rule, never believe any caller who says your computer is infected, and never download anything that's suggested. Microsoft and other legitimate companies don't do business that way. Ignore the fact that a caller knows your street address; it can be found in any land-line phone directory.

In the column you mentioned (see www.startribune.com/a1367), I wrote that the malicious software discovered by the FBI worked by redirecting the Internet browsing of infected computers to hacker-operated Web servers. The FBI continues to operate those servers as a way to tell consumers if their PCs are infected.

But, as you point out, the FBI's testing website (www.dns-ok.us) won't give the right answer if your Internet service provider is rerouting Internet traffic around the bogus servers.

So I asked Comcast and CenturyLink, the two biggest Internet service providers in the Twin Cities, whether they do any rerouting of Web traffic that might invalidate the FBI's test. Both say they don't. So, if the FBI page says your computer isn't infected, you can believe it. People with other service providers should ask their providers the same question.