Since yesterday Outlook started to stay in the foreground. If Outlook
is in the foregound (maximised or not) and I want to change to another
window with Alt-Tab, the new windows shows for half a second and then
Outlook is back.
Sometimes I succeed to get another to the foreground but as soon as I
click into that window Outlook is back again!
Any idea what could be the reason for that? I already did a reboot.

(Windows XP SP2; Outlook XP)