Since Windows XP logons are treated as terminal sessions, you can use the terminal services API to log users off. Something like this:

use strict;
use Win32::Lanman;

my $target = 'client01';
Win32::Lanman::WTSLogoffSession($target, 0);

# See also:
# WTSEnumerateSessions() 
# WTSQuerySessionInformation()
# WTSDisconnectSession()
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