Unable to access vSphere 4 webpage with IE (Firefox OK)

(Running ESXi 4.1)

Not sure when this issue first occurred, as a couple of months ago I directed a colleague to http://<servername> to grab the vSphere 4 Client download and that worked fine.

Now when I come to try it in IE 8, I get a "Internet Explorer cannot display the webpage" error. (Not even getting a warning page about the security certiticate)

Firefox is OK though (after initially accepting the certificate).

Can access via IE directly on server that vCenter is installed on using http://localhost (and http:/<servername> after I'd added it into IE's Trusted location)

Any ideas? Is it a recent IE update that's caused this? I've tried using the IP noth in IE and Firefox and found Firefox works, IE doesn't.

Thanks

[Edit: Just tested on a PC running IE6 and that's OK, so guessing security within later IE versions?]