Thanks Tamlin for the two useful links. Iknew that frames are not recommended, but I'm developping a small personal site which will contain very few pages and it seemed to me the easiest way (I don't want to spend too much time on it).
As for the fix, it works! Thank you so much! I had guessed it was an IE bug because I couldn't reproduce it on Firefox, but that didn't help me much
Thank you again.