i mean plain php pages
What actually your pointing with this line, plain php pages????
Any ideas as of what could result in such a straneg behaviour ?
1. Did you upgrade correctly, incorrect installation of PHP or MySQL could also cause these type of problems.
2. Find for Spywares , Trojans and viruses with your antivirus solution.
3. The Most common would be some type of malcious code sitting in your server and causing trouble for users when they are viewing your page using IE, the format is being changed.
Look for it too friend.