How do I prevent Windows Messenger from running on Windows XP?

Group Policy Method

On a Windows XP Professional-Based Computer;

Click Start , click Run , type gpedit.msc , and then press ENTER.
Double-click the following items to expand them:
Local Computer Policy
Computer Configuration
Administrative Templates
Windows Components
Windows Messenger
Double-click Do not allow Windows Messenger to run , and then click Enabled .
Click OK , and then quit the Group Policy snap-in.

Regedit Method

Windows Messenger 4.5 or Later on Windows XP Home Edition-Based or Windows XP Professional-Based Computers
Start Registry Editor (Regedit.exe).

Locate and click the following registry keys:

HKEY_LOCAL_MACHINE\Software\Policies\Microsoft
On the Edit menu, point to New , click Key , and then type Messenger for the name of the new registry key.

HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Messenger
On the On the Edit menu, point to New , click Key , and then type Client for the name of the new registry key.

HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Messenger\Client
On the Edit menu, point to New , click DWORD Value , and then type PreventRun for the name of the new DWORD value.

Right-click the PreventRun value that you created, click Modify , type 1 in the Value data box, and then click OK .

Quit Windows Messenger

This KB Article goes into details about this topic.