Was Symantec AntiVirus enabled when you installed IE7?
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1. Does this behavior persist if you start IE7 in No Add-ons mode?  To start
IE7 in No Add-ons mode:

Right-click on the blue IE desktop icon and select Start without Add-ons; or

Start > (All) Programs > Accessories > System Tools > Internet Explorer (No
add-ons).

Troubleshooting and Internet Explorer’s (No Add-ons) Mode:
http://blogs.msdn.com/ie/archive/2006/07/25/678113.aspx

2. Does the problem persist if you Reset IE7 Settings (RIES)?
http://support.microsoft.com/kb/923737 <= Read before using!

3. If you uninstall (a) IE7 (which automatically returns you to IE6), (b)
all toolbars and (c) all third-party Windows Themes (including Window
Blinds) and do NOT reinstall IE7, does this behavior persist?  If it does,
do NOT reinstall IE7.

Uninstalling IE7
http://www.ie-vista.com/kbase2.html

How to uninstall Internet Explorer 7:
http://support.microsoft.com/kb/927177

IE7 (Un)Installation and Anti-Malware Applications:
http://blogs.msdn.com/ie/archive/2006/10/11/IE7-Installation-and-Anti_2D00_Malwa
re-Applications.aspx

4. Required reading IMHO before reinstalling IE7:

Sandi's Installation Tips <= FOLLOW THESE TIPS TO THE LETTER!
http://www.ie-vista.com/known_issues.html#pre-install

I strongly recommend using
http://www.microsoft.com/windows/downloads/ie/getitnow.mspx as the
installation source, not Automatic/Windows Update.

Tip: Save the installer to your desktop, do not Run it.

Make *certain* that you check in at Windows Update immediately after
reinstalling IE7 to install any critical updates offered.  If Optional
Updates category offers Root Certificates update, I recommend installing it
to take full advantage of IE7's additional security.