Try looking in the Program options in Internet Explorer (Tools, Internet Options, Programs) and see which program is set as the default web page editor.
Have you fully cleared your temporary Internet files ?
Try clearing the cache as this sometimes, on some systems, affects the view source properties of the browser.
This is a solution I read about online but I've not tried it so I've no clue whether it works or not:
1. Close any instances of IE.
2. Create this registry key if it does not already exist (chances are it does not exist):
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\View Source Editor\Editor Name]
3. Modify the Default value of the "Editor Name" to the path of the program you want to use to view source code. For example, "C:\Program Files\Accessories\wordpad.exe" or "C:\Windows\notepad.exe"
I have my suspicions of that method.
An MSDN page for those who need to change registry settings for View Source click here
Microsoft Knowledge Base Solutions:
Hope one of them helps.