Mac wont shut down, restart or sleep - corrupted software update?

Question

My Mac Pro (2007 - OSX 10.6.7) will no longer shut down, restart or sleep. When I try to shut down, it logs me out of my user account, shows the light blue screen, momentarily shows the Leopard space background screen, before showing the blue screen again, and finally returning to the login screen prompting for my user/password.
The only way to poweroff from this point is to hold down the power switch until it forces a power down. This issue has been present for the last two weeks or so ever since I did a Software Update containing a security update. Initially it just failed to perform that update, although after a few days it did do the update, seemingly by fluke. After that, the problem persisted and now there is a new update (Fonts) which again fails to update.
I have tried all of the following:
Power off Mac, unplug power and leave for a few seconds
Power on Mac holding down Ctr + Opt + R + P (resetting PRAM etc)
Boot up from install CD and perform Disk repair (it did find problems with the disk and repaired them)
Check and repair permissions
Delete Software Update related plists & caches from ~/Library and /Library
Still the problem persists. Examining the logs shows me this:
04/05/2011 00:24:50          com.apple.WindowServer[11503]          Wed May  4 00:24:50 macpro.local WindowServer[11503] <Error>: kCGErrorFailure: Set a breakpoint @ CGErrorBreakpoint() to catch errors as they are logged.
04/05/2011 00:25:00          com.apple.launchd[1]          ([0x0-0x226226].com.apple.SoftwareUpdate[11517]) Job appears to have crashed: Segmentation fault
04/05/2011 00:25:00          com.apple.ScreenSharing.server[11508]          Wed May  4 00:25:00 macpro.local AppleVNCServer[11508] <Warning>: CGSShutdownServerConnections: Detaching application from window server
04/05/2011 00:25:00          com.apple.ScreenSharing.server[11508]          Wed May  4 00:25:00 macpro.local AppleVNCServer[11508] <Warning>: CGSDisplayServerShutdown: Detaching display subsystem from window server
04/05/2011 00:25:00          com.apple.UserEventAgent-LoginWindow[11507]          Wed May  4 00:25:00 macpro.local UserEventAgent[11507] <Warning>: CGSShutdownServerConnections: Detaching application from window server
04/05/2011 00:25:00          com.apple.UserEventAgent-LoginWindow[11507]          Wed May  4 00:25:00 macpro.local UserEventAgent[11507] <Warning>: CGSDisplayServerShutdown: Detaching display subsystem from window server
04/05/2011 00:25:00          com.apple.ReportCrash.Root[11518]          2011-05-04 00:25:00.895 ReportCrash[11518:2703] Saved crash report for Software Update[11517] version ??? (???) to /Library/Logs/DiagnosticReports/Software Update_2011-05-04-002500_localhost.crash
04/05/2011 00:25:02          com.apple.WindowServer[11520]          Wed May  4 00:25:02 macpro.local WindowServer[11520] <Error>: kCGErrorFailure: Set a breakpoint @ CGErrorBreakpoint() to catch errors as they are logged.
I can paste the crash report referred to in the logs if that would be helpful but for now will leave it out as it's huge and largely full of gobledygook.
I'm at my wits end now. I've always been very proud that in ~4 years of Mac ownership I've never had to reinstall OSX - something I used to do on an anual basis with Windows. My next step probably is a full reinstall, but I really do consider it a drastic step and a last resort.
Here's hoping that the Mac Support Community can help me out
Thanks in advance.

Answer 1

As a followup, upon the advice from someone else I created another user account (non admin) and logged in as that user. From that account I could successfully power down.
I'm still convinced the issue is related to Software Update as examining the logs showed none of the Software Update related entries as above:
04/05/2011 09:56:40          com.apple.WindowServer[760]          Wed May  4 09:56:40 macpro.local WindowServer[760] <Error>: kCGErrorFailure: Set a breakpoint @ CGErrorBreakpoint() to catch errors as they are logged.
04/05/2011 09:56:44          com.apple.coreservicesd[87]          ThrottleProcessIO: throttling disk i/o
04/05/2011 09:56:50          com.apple.loginwindow[759]          2011-05-04 09:56:50.297 migCacheCleanup[781:903] Flushing Cache Locations...
04/05/2011 09:57:12          com.apple.WindowServer[840]          Wed May  4 09:57:12 macpro.local WindowServer[840] <Error>: kCGErrorFailure: Set a breakpoint @ CGErrorBreakpoint() to catch errors as they are logged.