(Note: This is in reply to Ron's post. Unfortunately, if I'm not copied 
as a separate recipient it doesn't seem I can really thread replies 
properly, without subscribing to the list and inundating my inbox anyway.)

I had actually user-started a Process or three recently, but this seems 
an unlikely culprit:

1. The hang coincided *exactly* with the start of the virus scan; I'd
    been using [foo] newProcess resume for a bit without problems other
    than with non-reentrancy of blocks.
2. Said processes actually being started with [foo] newProcess
    setPriority: Processor lowestPriority; resume anyway. (Or maybe just
    priority: or whatever; don't recall offhand.)
3. Task manager showed (and still shows) the hung Squeak process using
    0% CPU. A Process hogging the CPU away from the UI handler would
    saturate it instead. Instead the UI event handling thread seems to
    have been starved, or begun blocking until a condition that is never
    actually met. This smells like starvation or a deadlock to me, and it
    seems to involve the interaction with native multitasking. Perhaps
    I/O blocking for long enough risks provoking a latent
    deadlock-causing bug at the low levels of the Windows implementation;
    a virus-scan causes heavy contention for disk I/O.