If you upgraded your computer from Windows NT 4.0, issuing an IPCONFIG / RELEASE may halt your computer and/or the CPU usage approaches 100%.
This problem will occur if you had the full version of Network Manager, version 1.2, installed prior to the upgrade.
If you can get to a CMD prompt, type:
NET STOP NMAGENT
To resolve the problem, uninstall the NMAgent service and install Network Monitor version 2.0.