IE7 in Vista — How to Configure Your View Source Editor
It seems as though Microsoft has implemented a new security feature for Microsoft Vista users to makes opening up a View Souce window much more difficult than on any other platform. On older Windows platforms, you would just left-click to open the menu, then select view source and voila it would open in notepad.
Windows Vista includes an IE security feature called Protected Mode. Protected Mode runs the IE process with lower privileges and also helps protect against malicious webpages that try to automatically pass content to higher privileged applications like Notepad.
This prompt is designed for the worst case security scenario, which is a malicious webpage trying to silently elevate out of Protected Mode by launching an application or reusing one that you’re launching. For example, in the scenario where you select View Source, a malicious webpage could try to silently pass its content to Notepad instead of the webpage’s source code. This could be a dangerous scenario if there was vulnerability in Notepad.

If you only browse to web sites you trust and you don’t want to click through this prompt in the future, you can check the “Do not show me the warning for this program again” box before clicking “Allow”. Checking this box and “Allow” will add the following entry to Protected Mode’s elevation policy:
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Low Rights\ElevationPolicy\{9F5511FE-4BB1-474D-B6ED-8877567E7F36}]
“AppPath”=”C:\\Windows\\System32″
“AppName”=”notepad.exe”
“Policy”=dword:00000003
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