A weird problem - In Calendar view the checkboxes next to tasks are greyed-out so the task can't be ticked to mark it complete. I manage 20 odd identical PC's and this only applies to mine! I've gone through all the options and can't find anything relevant. Also tried opening a second PST file - the calendar view in that one was OK so maybe the problem is connected with the PST?
Obviously you can use right-click for the pop-up menu and use that to mark complete but it's a nuisance and I would be interested to hear if anyone else has come across this and, hopefully, found a solution!