This option does not exist in any client yet... [don't ask me why]. It's driven by the preset profile Power Saving btw, which is the only one that has this field populated to kick in with a default of 20 minutes.

I have tried the solution specified above, but the suspension is still happening for no apparent reason.
Below is an excerpt from my msg file...

15-12-2009 12:00:56||Starting BOINC client version 6.2.28 for windows_intelx86
15-12-2009 12:00:56||log flags: task, file_xfer, sched_ops
15-12-2009 12:00:56||Libraries: libcurl/7.19.0 OpenSSL/0.9.8i zlib/1.2.3
15-12-2009 12:00:56||Data directory: C:\Documents and Settings\All Users\Application Data\BOINC
15-12-2009 12:00:56||Running under account kor
15-12-2009 12:00:56||Processor: 2 GenuineIntel Genuine Intel(R) CPU 2160 @ 1.80GHz [x86 Family 6 Model 15 Stepping 2]
15-12-2009 12:00:56||Processor features: fpu tsc pae nx sse sse2 mmx
15-12-2009 12:00:56||OS: Microsoft Windows XP: Professional x86 Editon, Service Pack 3, (05.01.2600.00)
15-12-2009 12:00:56||Memory: 3.49 GB physical, 4.80 GB virtual
15-12-2009 12:00:56||Disk: 78.13 GB total, 45.69 GB free
15-12-2009 12:00:56||Local time is UTC +1 hours
15-12-2009 12:00:56|World Community Grid|URL: http://www.worldcommunitygrid.org/; Computer ID: 1129507; location: (none); project prefs: default
15-12-2009 12:00:56||General prefs: from World Community Grid (last modified 10-Dec-2009 16:08:29)
15-12-2009 12:00:56||Host location: none
15-12-2009 12:00:56||General prefs: using your defaults
15-12-2009 12:00:56||Reading preferences override file
15-12-2009 12:00:56||Preferences limit memory usage when active to 1787.62MB
15-12-2009 12:00:56||Preferences limit memory usage when idle to 2681.43MB
15-12-2009 12:00:56||Preferences limit disk usage to 9.31GB
15-12-2009 12:02:56|World Community Grid|Restarting task HFCC_s1_01061956_s1_0000_0 using hfcc version 610
15-12-2009 12:02:56|World Community Grid|Restarting task mz445_00058_13 using hpf2 version 603
15-12-2009 12:30:06||Suspending computation - no recent user activity
15-12-2009 12:30:19||Resuming computation
15-12-2009 12:35:14|World Community Grid|Sending scheduler request: Requested by user. Requesting 0 seconds of work, reporting 0 completed tasks
15-12-2009 12:35:19|World Community Grid|Scheduler request succeeded: got 0 new tasks
15-12-2009 13:13:47||Suspending computation - no recent user activity
15-12-2009 13:17:30||Resuming computation
15-12-2009 13:37:49||Suspending computation - no recent user activity
15-12-2009 13:42:24||Resuming computation
15-12-2009 16:13:24||Resuming computation


Hope this help to find a solution, if you need more info pls let me know!!!
kkoo

Also, indicates that you're using your local preferences file. Thus, I'd recommend clearing that for now, so that we're just using the preferences from the Server.

To clear these settings, open up your BOINC session in Advanced View, then select the "Advanced" pull down menu, then "Preferences", click the "Clear" button, and finally, click the "OK" button.

plz go back to Snow Crash's post on where to change the mouse/keyboard idle setting, on the website!!!

edit: So go My Grid > Device Manager > Device Profiles > Default > Custom

Stop work after computer is idle for: xx minutes

Save and do an update in the client project view.