At shutdown, I consistently receive the following error code:

"0X00000000" reference memory at "0X00000000" This memory could not be
written. Click OK  or Click Cancel to Debug.

When I click OK, the system shuts down; however, sometimes it restarts
rather than shuts down.

System: PG4 3.2 GHz, 2 GB RAM, NVidia 6800 Ultra, 200 GB RAID 0 Windows XP Pro SP2.

on the top of that error box, does it give you a .exe file name, ex:
program.exe, and then the error on the screen

Thanks for looking into this.

Yes, the error window is titled explorer.exe and I think it also has
"execution error"

The error code is then in the window

Hi,
That type error is usually caused by a spyware install ...... 
Check for Spyware | Adware | Parasites | Dialers | Hijackers | Trojans: 
[Experienced Users] 
SpyBot 1.2  [freeware] http://security.kolla.de/ 


Once installed make *sure* to update via online before scanning! 
Fix the items labeled in red,  items labeled in blue-green are optional. 
Support Forum: http://www.net-integration.net/cgi-bin/forums/ikonboard.cgi 
How To: http://www.tomcoyote.org/SPYBOT/ 


[Inexperienced Users] 
Ad-Aware [freeware] http://www.lavasoftusa.com/ 
Once installed make *sure* to update via online before scanning! 
Support Forum:http://www.lavasoftsupport.com/ 


[To double-check your system] 
Go to: http://www.tomcoyote.org/hjt/ 
Download "Hijack This!" [freeware] or download direct (below): 
http://www.spywareinfo.com/~merijn/files/hijackthis.zip 


Unzip, double-click "HijackThis.exe" and Press "Scan". 


When the scan is finished, the "Scan" button will change into a "Save Log" 
button. 
Click: "Save Log" (generates: "hijackthis.log") 


Next, go to the below location: 
http://www.spywareinfo.com/forums/ 


Sign in, go to the "Spyware and Hijackware Removal" section. 
Press "New Topic", copy and paste both files into your new message.