XP: Weird! Moving Folders on Desktop Always Launches Odd Installer!?

This is very weird! I'm running XP Pro SP3, and, all of a sudden, every time I try to copy or move a folder on the desktop, an application installer runs!

The explorer operation works fine, but the installer always runs too.

I'm not sure it matters, but in this case the installer it tries to run every time is the Windows version of Smith Micro StuffIt 2009. That app is already installed (I installed it a year ago), and both the app and the installer are reported to be malware-free.

Naturally, it seemed to me that this must be a malware infection of some kind, so I ran everything at my disposal with maximum settings and the latest updates: Avira AntiVir 2010 Premium, SuperAntiSpyware Pro, and Spyware Doctor on every file on every disk. I turned OFF any settings that limited their scans just to the standard executables and archives so that they'd all scan every single file, no matter what it's extension. None of them found any infections of any kind (I'm careful to try to keep my systems clean).

But it also seems to me that an infection would attempt to do something harmful to my system, not merely try to re-install an already installed app. But I realize there are bugs even in malware, so who knows?

I didn't install anything recently or even knowingly download any new files. All I've been doing for the last few days is browsing and using email.

I looked at the explorer.exe registry settings and nothing looked as if there were some hook to the StuffIt installer, but then again I don't know exactly what those registry entries should look like.

Any suggestions on what I should look at or try next?