Okay, guys...time out!
It's rare for "system" to run at 100% CPU.
Normally, when I see it happen, it's usually during a file copy operation over the network.
In any case, try looking in the Event Viewer:
Start -> Run -> eventvwr -> press Enter -> check Application and System for any red X's.
Also, looking at your Process Explorer results, it's going to be one of the "subprocesses" under "System."
In this case:
System 4 100.00
smss.exe 372 Windows NT Session Manager Microsoft Corporation
csrss.exe 528 Client Server Runtime Process Microsoft Corporation
winlogon.exe 556 Windows NT Logon Application Microsoft Corporation
services.exe 600 Services and Controller app Microsoft Corporation
ati2evxx.exe 756 ATI External Event Utility EXE Module ATI Technologies Inc.
svchost.exe 768 Generic Host Process for Win32 Services Microsoft Corporation
svchost.exe 872 Generic Host Process for Win32 Services Microsoft Corporation
svchost.exe 936 Generic Host Process for Win32 Services Microsoft Corporation
wuauclt.exe 2020 Automatic Updates Microsoft Corporation
svchost.exe 1028 Generic Host Process for Win32 Services Microsoft Corporation
svchost.exe 1240 Generic Host Process for Win32 Services Microsoft Corporation
ccSetMgr.exe 1420 Symantec Settings Manager Service Symantec Corporation
ccEvtMgr.exe 1448 Symantec Event Manager Service Symantec Corporation
ccProxy.exe 1528 Symantec Network Proxy Service Symantec Corporation
spoolsv.exe 1656 Spooler SubSystem App Microsoft Corporation
navapsvc.exe 1896 Norton AntiVirus Auto-Protect Service Symantec Corporation
wdfmgr.exe 192 Windows User Mode Driver Manager Microsoft Corporation
symlcsvc.exe 2080 Symantec Core Component Symantec Corporation
SNDSrvc.exe 2328 Network Driver Service Symantec Corporation
NSCSRVCE.EXE 3484 Norton Security Console Norton Protection Center Service Symantec Corporation
lsass.exe 612 LSA Shell (Export Version) Microsoft Corporation
ati2evxx.exe 1176 ATI External Event Utility EXE Module ATI Technologies Inc.
Since there are an awful lot of Symantec processes there, Broni could very well be correct.
Norton (consumer-grade Symantec) products do have a reputation for being bloated. Business-grade Symantec products (Symantec Antivirus Corporate Edition, Symantec Endpoint Protection) don't seem as bad, though.
Try uninstalling Norton. Best case scenario, the problem is solved and you can look towards alternatives for anti-virus and software firewall protection. Worst case scenario, that doesn't solve the problem and you can reinstall your Norton stuff if you wish.
