The standard version of Windows messnger did not have the option for a
SIP account, so I uninstalled it, and installed 5.1 which I confirmed
on a 2003 server machine did have sip enabled. I have uninstalled, and
rebooted, and re-installed version 5.1 multiple times with no success
of allowing the option for a SIP account.

I even resorted to uninstalling, re-installing and then installing SP2
still to no avail.

Details:

Windows XP SP1 (now SP2)
Windows Messnger 5.1
No option to use a SIP account is available on the Accounts tab.

If more details are required please let me know and I will be happy to
provde them.