I spent several hours this weekend fiddling with Windows instead of playing. Somehow my Internet Explorer had become borked, showing only pages with basic HTML, and being unable to show many pages using more advanced code. The culprit was probably mshtml.dll, or another library. Anyway, I wanted to fix the problem by reinstalling IE7. But to install IE7, the installation program insists on running Windows Updater. And surprise, surprise, Windows Updater uses Internet Explorer. Using Windows Updater in the tools menu of IE just sent me to an empty page.
Fortunately parts of the Microsoft Download site still worked. So in the end I was able to fix the problem by downloading an .iso file of Windows Service Pack 3, burning it to CD, and installing it from there (as I obviously couldn't install it with the standard method of using the Windows Updater). After that I was able to reinstall IE7, and now everything works fine. Checked the drive for viruses, in case it was that which borked my files, but found nothing. So it was just a normal Windows SNAFU. Sigh!