Right! Ive found a workaround for anyone who is interested. Microsoft can't force Windows 9x/2000 users to upgrade to Windows Live Messenger, because the program hasnt been released for it.
Go into the MSN Messenger folder in Program Files, right click on the MSN logo, select properties.
Go into the "compatibility mode" tab, and set it to run in compatibility mode for Windows 2000.
This then fools MSN Messenger and the MSN servers into thinking you are running it on Windows 2000 instead of XP, and therefore it can't force you to upgrade.
Not sure how long it will work for until Microsoft realise it, or if it works after a reboot, but its been good for me so far!
Up yours Microsoft, you aint taking my beloved MSN Messenger 7.5 away from me and making use that pathetic pile of pig crap aka Windows Live Messenger that easily!!!