if its any consolation the one i am now is XPP, and just downgraded one omy laptops from Vista Ultimate XPP.. the wife has the only W7 machine.. (also run a server with server 2003). one desktop is 'clean' with no security on it and has never been probed, but being clean, there's chuff all on it anyway.
so the XPP one with IE 8 works fine... i'm securitied up the eyballs two software firewalls and one hardware and run through two subnets (and freak the IP address as well from time) (isn't paranoia but necessity)
if the O/S they work through are on the same network with the same security it probably isnt port blocked (but if the security settings on that one are different thay may cause it)
you could disable all security and see if that works
you may have some random issue with a registry entry somewhere.. download ccleaner
www.ccleaner.com , run that and the reg clean function on it and see how that works...