I had to call Lenovo tech support to get this unpublicized fixed, but I was relieved they at least had a fix. I asked the support tech specifically to check for notices mentioning Lenovo freezes. I was on hold less than two minutes before support came back on the phone with the culprit and a solution.

It turns out that the Realtek High Definition Audio driver that ships with WinXP version of Lenovo 3000 N100 laptop (possibly other models, too) was causing utilization spikes that made the laptop freeze periodically. Their solution was to update to driver version 5.10.0.5366 or later. The driver is available on Lenovo’s site by searching for support article
MIGR-66525.

I downloaded the update and followed the online instructions for loading: run the *.exe file to extract installation files, browse to where installation files are saved, and then run the AdDrvLdr.EXE file. Accept defaults to update audio driver, and then reboot. It worked; no more freezes.