This leaves the icons on your computer but when clicked an error message appears stating file not found. This will make your bro think the program is damaged. If he reinstalls it simply repeat the process...eventually he'll get tired of installing "that m/s crap" and you're all set.
Good luck

First, right click the Messenger icon in the system tray and click Exit.
Right click the Start button and click the Explore command to open the Windows Explorer.
In the Windows Explorer, expand the Program Files folder on the partition where your operating system is installed (the partition that holds the WINDOWS directory) and right click the Messenger folder and select Rename.
Rename the folder to MessengerDISABLED and press ENTER to commit the name change.
Restart the Computer. No more Windows Messenger!
A number of people said they couldn't rename the folder, because they got a message that said there was a file in use. If this happened to you, try this:

Right click the taskbar and click the Task Manager.
In the Task Manager, click the Processes tab.
On the Processes tab, click the Image name column header. You'll notice that this arranges program names in alphabetical or reverse alphabetical order. This prevents them from moving around in the list and makes it easier to find what you're looking for.
Find the msimn entry in the list. Right click it and click End Process. Click Yes in the dialog box warning that bad things might happen. The msimn entry should disappear.
Find the msmsgs entry in the Image Name list. Right click that one and click End Process. Again, click Yes on the warning dialog box.
Close the Task Manager.
Now that both the Windows Messenger and Outlook Express are closed completely, you should be able to rename the C:\Program Files\Messenger folder. For info on other ways to whack Windows Messenger, look in the Troubleshooting section at the end of this newsletter for a link to Microsoft's official methods.


source: http://www.winxpnews.com/index.cfm?id=80