My wife spent last week rebuilding her computer's software from the ground up, starting with a drive reformatting and complete OS reinstall. Her troubles started soon after we connected to the DSL network in Shanghai.

When my wife and I lived in Beijing we our Internet service was provided by an operator affiliated with our apartment complex. The entire building was wired with a LAN. For a nominal fee you could plug in and go. It had some drawbacks, however. Most annoyingly, the leased line that served the building didn't seem especially fast, and at peak hours the whole thing was prone to grind to a near halt. It also suffered from fairly regular service interruptions.

I always assumed the whole network was a swamp of malware. Chinese networks are notorious after all. But neither my wife's PC nor my Mac, both of which run the usual range of security software and live behind a wireless router with a firewall, ever suffered any serious problems.

In Shanghai, I use DSL, provided by Shanghai Telecom, the local China telecom affiliate. It's consistently faster than the old Beijing building LAN. It works great with the wireless router and talks nicely to my Mac and my wife's PC. But it didn't take long after we connected for my wife's troubles to begin.

The first sign of trouble was a nagware window. This was not a typical click-and-close nagware window. It could not be closed and it dominated the entire center of the monitor, always staying on top. It was also completely garbled and unreadable. My wife's PC is old-ish and runs Windows 2000. It has always had trouble displaying Chinese characters in software UIs. Web pages are not a problem, but this window was a bunch of gibberish with an input field an image showing a receipt with a circled number.

It started the same day our first bill arrived, so at first we thought it was bill payment nagware. That really annoyed us. No grace period? So my wife went and paid the bill a day or two later and the window vanished, apparently confirming our suspicions.

But over the next couple of days my wife's PC began a slow disintegration. It began when the nagware window appeared again. Then her McAffee security software started detecting a buffer overflow in Internet Explorer, even as more and more Chinese ads started appearing in the browser. Then her AIM Messenger software started persistently trying to launch itself. No amount of scanning or scrubbing with McAffee or AdAware seemed able to help. In just a couple of days the computer was all but unusable and my wife conceded defeat. The great rebuilding began.

After rebuilding, and switching to ZoneAlarm's security suite, there has been no further problem. My wife called Shanghai Telecom and it explained that the indecipherable nagware window had actually been a warning from its network that my wife's computer was compromised by a Trojan. However, it didn't have much to say beyond that. My wife suspects the Trojan came through AIM and she accidentally enabled it to install by clicking the "OK" button on an install dialog masquerading as a Microsoft software update window. None of her software was able to stop it.

My wife is planning to buy a new computer this autumn, so it doesn't seem worth paying for a new operating system at this point. Her aging computer probably won't even run Vista, although she did add RAM to support the extra security software she is now running.

The lesson? If you're going to connect to a swampy Chinese network, run the hardest version of Windows and the best range of security software you can reasonably support. And stay away from the mystery dialog boxes.

Or, do what I do and, in addition to the common sense rules above, use a Mac. Two years later and still trouble-free.