What is so ironic is that Windows Live Messenger is fully compatible with XP x64, as a matter of fact I’m using it right now. So what you have to do is download the file I mentioned that the unsupported installer downloads. Take note though that you will not get any prompts during the install, it is silent, since you are supposed to make those choices in the unsupported installer… What this means is that you are saved the trouble of unticking the boxes about installing MS toolbars and changing your browsers home page to the dreaded MSN sites.
Also worth mentioning is that Windows XP x64 shares its codebase with Windows Server 2003 x64 and even uses the same service packs!) So this should apply to Server 2003 as well, just like drivers for either OS work fine with each other.
You can download the bare MSI file of Windows Live Messenger v8.5.1302.1018
here. While you are at it check out
A-Patch that allows you to disable many annoying features of MSN like advertising - it’s beyond me why MS advertises in their own client. And when you have come to your senses you can may as well download a decent chatting client instead

.
Oh, and here is the command to uninstall the Windows Messenger (not the same as MSN Messenger OR Live Messenger) that is bundled by default with the OS. Just copy and paste the following into Start > Run and Windows Messenger will be gone:
RunDll32 advpack.dll,LaunchINFSection %windir%\INF\msmsgs.inf,BLC.Remove