ntldr in use by another application?

I've been trying unsuccessfully - five times now, both online and via
automatic updates, both express and custom installs - to install SP2. As
setup is inspecting my system, it tells me that ntldr is open or in use by
another application. No other application is open in the Task Manager window.

I have McAfee enabled at startup, but have both disabled each of the (online
version) applets individually, then exited from the icon in the systray, and
have just exited from McAfee without disabling the individual applets. I have
also ended the individual processes and trees using TM (mcv*), still to no
avail. With McAfee Virus Scan now running (I'm just checking everything to be
sure), this is the list of the processes now running following another
unsuccessful SP2 install attempt (all have .exe extensions unless indicated
otherwise; those that appear twice or more are also shown):

mcmnhdlr
iexplore
taskmgr
cidaemon
cidaemon
apcsystray
qttask
mscifapp
McVSEscn
mcvsshld
type32
point32
realsched
INSTAN~1 (?!?)
inetinfo
qbupdate
wdfmgr
svchost
snmp
SZQKPICK (?!?)
tcpsvcs
wuauclt
MpfService
mcvsrte
cisvc
mainserv
spoolsv
explorer
MpfAgent
svchost
svchost
svchost
svchost
mcagent
lsass
services
winlogon
csrss
smss
McShield
mghtml
Acrotray
DvzIncMsgr
System
System Idle Process

Any good guesses?