I had a problem that drove me CRAZY for over a week.
In Windows Vista "and probably Windows 7" you have the option to dispay a web page in "compatibility mode" in case the web page was designed for XP or earlier.
The button to do this is on the right side of the browser window directly to the left of the refresh button. I had mistakenly put my pc in compatibiliy mode and it did exactly what you are describing.
Hit the compatibility button one time and see if that fixes is for you. If you can't figure out where it is let me know and I'll do a screen shot showing you where it is.