Hanging On I/O

Windows XP SP3 with all critical patches include the twenty some odd from last Tuesday. (Note the
problem has occurred before Tuesday's updates.)
No Domain Server -- Using Workgroup on a small LAN
Office 2007 Ultimate
Using a userid that is a member of the Administrators' group.

Every now and then it appears that a disk I/O hangs. It does not affect programs that are running
(i.e. Outlook 2007) but does stop Explorer (or any browse that uses the open/save file dialog) from
starting. The symptoms include the disk activity light staying on solid (which is the reason I
think it is a hung I/O request.

I do not have any mapped network drives so I don't think it is a network problem although my profile
is setup to look like a roaming profile by setting it to a local share. Home directory points to a
directory on a local driver. There is a subst to point H:\ to a directory on a local drive.

I have replaced the EIDE drive that contains the operating system and the user partitions and that
has not cured the problem. There are two internal SATA drives and a SATA drive in an Addonics
Mobile Rack. There are also four external USB drives connected to the system. There is also an IDE
DVD burner and a virtual CD drive (Virtual CD which is certified for XP and which I have used for
years). I can't have any of the external drives disconnected for an extended period so trial and
error seems to be out as a way of isolating the problem.

When the problem occurs I can usually logoff. During the last logoff process the system asked to
kill Explorer and ctfmon.exe so I suspect one of them is hanging on an uncompleted I/O or is trying
to enque a resource that is already in use.

I have check the event logs and they only contain the standard information messages around the time
of the hang. The only warning in the system event log was seventeen hours before the last hang:
Event Type: Warning
Event Source: MRxSmb
Event Category: None
Event ID: 3019
Date: 10/19/2009
Time: 10:24:17 PM
User: N/A
Computer: NYSABW0007
Description:
The redirector failed to determine the connection type.

Is there any utility I can run in background that will let me know what is blocking the system when
the problem occurs. (It needs to be already running as I may not be able to start it once the hand
occurs.)