outlook calendar

I have a recurring problem with my outlook calendar. I use windows xp. After
a varying number of times I get a message when I try to access my calendar
saying it is in use by another program and cannot be accessed. It then opens
in the archive window. No one else in my network accesses my calendar.
If I shutdown and restart it fixes the problem. However I may then encounter
another problem and have to do that again. I use my calender frequently and
need reliable access. Can anyone help?
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maggie d