To prevent Outlook 2002, Outlook Express 6, Remote Assistance, and other programs that may use the Windows Messenger APIs from using Windows Messenger, you can enable the Do not allow Windows Messenger to be run Group Policy at / Administrative Templates / Windows Components / Windows Messenger. This setting is available in both User Configuration and Computer Configuration, with Computer Configuration taking precedence.
If you do not configure group policy, you can use REG.EXE, built into Windows XP, to run
REG ADD HKLM\Software\Policies\Microsoft\Messenger\Client /V PreventRun /T REG_DWORD /F /D 1
for each client.