Looks like you’re gonna have to use an alternate method, like ProxHTTPS (or whatever it’s called) or downgrade to 1.0 through 4.7.
If 4.5-4.7 (escargot’s downloads, NOT the one built in with XP SP3. The reason I say this is because it works simular to 5+, and uses a slightly more messy way of connecting with https and all that) doesn’t install, open the certificiate menu you used to check if 'check root certifites and uncheck Windows Messenger, then try again.
If ProxHTTPS doesn’t work, you might as well upgrade to Vista or 7.