Can't install WinMsgr 5.1

I am trying to install Windows Messenger 5.1 on my office PC. I've confirmed
that this is allowed under my account, and have seeked help from our IT
department. I am running Windows XP SP2 (Build
2600.xpsp_sp2_gdr.050301-1519), but when I run the messenger.msi file it
tells me 5.1 cannot be installed because I don't have the proper version.
I've checked the support database, but can't find anything that would help me
with this. Does anyone have any insight as to what to do?

Many thanks,
Ed S.
.