Here's one possibility:

Error message after you install a Windows Internet Explorer 7 update from
Windows Update or from Microsoft Update: "Webpage cannot be displayed":
http://support.microsoft.com/kb/942818

If tweaking your third-party firewall's settings don't help, try the
following:

1. Disable your anti-virus application, all Norton Add-ons (if any), any
real-time protection provided by an anti-spyware application (except for
Windows Defender; includes Spybot Tea Timer, SpySweeper Guards, AVG AS
guard.exe, etc.), disable any third-party firewall AND THEN enable the
Windows Firewall.

NB: You're "working without a net" now: Don't to ANYTHING else online (e.g.,
browsing; reading email; chatting) except #4 below until your anti-virus
application has been re-enabled!  If you're running ZA Free firewall, you
may need to uninstall it instead of disabling it.

2. Uninstall "Security Update for Windows Internet Explorer 7 (KB950579)"
via Installed Programs (make sure Show Updates box is checked at top right)
and reboot.

3. Re-do #1 above.

4. Run Windows Update manually and install any Critical Updates
offered.  If  a Root Certificates update is listed in the Optional Updates
category, install it to take full advantage of IE7's enhanced security.

5. Reboot > Make sure everything in #1 above is enabled again and, if
applicable, disable the Windows Firewall and enable (or reinstall) your
third-party firewall.