Application-dependent internet speeds

Hi folks,

I'm running Vista RTM and finding some strange internet speed behavior. I
have a web app hosted by a commercial hosting provider and my speeds in
accessing the site and the email accounts run as follows;

1. IE on Vista: web access is quick
2. Firefox and Opera on Vista: web access is slow (approx 1 kbps)
3. Outlook 2007 and Windows Live Mail Desktop on Vista: email access is slow
(approx 1 kbps, but normal speed for all other mail accounts).
4. Outlook 2007 (and previously 2003) on WinXP: email access is quick for
all mail accounts.
5. IE, Opera and Firefox on WinXP: web and mail access are quick (this is
even true when XP is running as a Virtual PC machine within Vista).

What gives? I don't understand why one app should be so significantly
faster than another. Any ideas how I can influence this?

cheers,
Ben