taskid 124684729 - can't acquire lockfile

(same happened on 124637804)
Some background info which might help.

windoze xp home sp3
I run Boinc (e@h only) on my everyday LUA id only.
Logged off my LUA id while 124684729 was running (so Boinc shut down).
Logged on to admin id and did some updates/housekeeping. Rebooted and logged on to admin for some tidying up. Logged off, logged on to LUA. Boinc started as normal, eventually 124684729 crashed with 'can't acquire lockfile'.
Dug around a bit. In stdoutgui.txt,

09:17:49: Error: can't open file 'C:\Documents and Settings\mikeg\My Documents\BOINC\\RebootPending.txt' (error 2: the system cannot find the file specified.)

If here's anything else which might be in the Boinc files and could be useful, just ask.