Spinning blue ring since SP1

T7700, 4GB, Vista Business
Avast! and Defender

This ran very well before SP1, but since then I've had
ring problems.

Right-clicking on a desktop icon sometimes evokes that
ring for quite a while. Any click on an email message or
some other icon might do the same. I cannot reliably
cause the problem with some set of conditions. Once the
problem arises, however, impatience leads to a white
screen and Explorer reload.

Various malware scans found no problems. I've turned off
UAC, checked with ShellExView, and found nothing.

In all other respects the unit is working flawlessly and
fast. Very fast.

???