May be the reason.
When you try to shut down or to restart your Windows XP-based computer, the shutdown processing may stop. This problem may occur if all of the following conditions exist:
You use the Fast User Switching feature.
Only one user is logged on (except for session 0).
Windows XP is running under heavy stress.
This problem occurs because messages that are sent to Winlogon remain in the Winlogon message queue. This blocks further processing.
To resolve this problem, obtain the latest service pack for Windows XP.
For additional information, click the following article number to view the article in the
Microsoft Knowledge Base:
322389 How to Obtain the Latest Windows XP Service Pack