I noticed that for the last few months that IE 6.0 has stopped letting me view source on webpages. I finally did something about it today, if you are having the same problem follow this advice:

1. Make sure notepad.exe is still located in your \Windows directory and is working right. If it is missing or got replaced by a 3rd party app, restore it from your Install-CD. The View Source command does tie into Notepad.exe, with Notepad missing or being defective, it can't work.
2. In Internet Explorer, go to Tools|Internet Options|General and delete the Temorary Internet Files. While you're there, make sure you Temporary Internet Files folder isn't too big, we noticed the problem occurs more often the bigger our folder was. Reduce it in size if you have this problem frequently.
3. If that is still not working, go to the \Windows\Temoprorary Internet Files folder on your harddrive and delete the files manually - incl. the index.dat file.

These steps cleared up the problem for me, at least temporarily.