From the page Bob linked to:
Web Site Connectivity Issues
SSL Changes
When navigating to an HTTPS site on Windows Vista, you may encounter a connection failure error page if the server does not correctly support the TLS standard. To work around this problem, uncheck the “Use TLS v1.0” checkbox in the Internet Control Panel’s Advanced tab and notify the web site owner about this problem. (A description of the changes in SSL can be found in the Internet Explorer blog).
Proxy Connectivity
If you notice slowness or a problem connecting to web sites after upgrading to Internet Explorer 7 Beta, it may be due to Proxy Auto Detection problems.
Launch the Internet Control Panel by clicking "Internet Options" From the Internet Explorer Tools menu.
Navigate to the Connections tab
Under Local Area Networks (LAN) settings choose LAN Settings
Uncheck "Automatically detect settings" in the Local Area Network (LAN) Settings dialog,
Check "Use a proxy server for your LAN" and specify your proxy server in the Local Area Network (LAN) Settings dialog,
Click OK, OK
You will now be able to navigate to internet sites.
Internet Connection Settings
If you notice a problem connecting to web sites, it may be because the upgrade to Internet Explorer 7 changed your Internet Explorer 6 Internet Connection settings. To work around the problem:
Launch the Internet Control Panel by clicking "Internet Options" from the Internet Explorer Tools menu.
Navigate to the Connections tab
For each connection in the "Dialup and Virtual Private Networks" list, click the "Settings" button
Verify that "Automatically detect settings" is checked. If not check it
Click the "Lan Settings" button in the Connections tab, and repeat step 4 6. Click OK on the Connections tab.
Close IE and restart it.