Consistently.
WinXP pro all patches, P4, Latest DX (9b now I think), Visiontek 4600 128 vram, plenty of RAM and so on.
EDIT:
Upon restart of SL, the "SL is already running, check for minimized..." error dialog appeared.
Inspecting running processes in task manager showed no instances of SL, newview, or anything else I didn't recognize, running.