My first suggestion is to
get Firefox, which has a built-in source viewer instead of using an external application.
Anyways..
1) if you have a shortcut to Notepad anywhere (not just on your desktop) delete it
2) empty your cache (Tools->Internet Options->General->Delete Files)
If these do not work, I found a lot of other opinions when I went to:
http://www.google.com/search?q=ie+cant+view+source
good luck ;)
-Snib -
http://www.snibworks.com
Where will you be in 100 years?