I confirm the problem on notebook Acer 6460, Windows XP Prof. international, Office 2007, with all patches and fixes available installed.
No problems exist in Word or any other application, be it Microsoft or other.
Only in Outlook 2007, I cannot type diacritics with capital letters (Č, Ď, Ú, ...). For lower case letters, the problem is less universal: I cannot type ň, ť, ď, but I can write all others (ě, š, č, ř, ... ). However, sometimes it depends on the context: either I cannot insert ř in a word and I can do so only after I insert a space and write at least one normal letter before the new letter ř, or I must re-type the previous letter. The problem, as far as I remember, never happens with é, í. To make the problem even more complex, I remember that immediately after some Microsoft update approximately 6 months ago, I was able to write any of these letters, but this bright period was ended by the next rebooting of the computer.
I have the same basic software on another PC and there is no problem with diacritics whatsoever.
ZS2
