You probably have Word's overtype enabled.

refer he
How to turn off or disable the Overtype mode in Excel 2003, in Outlook 2003,
and in Word
http://support.microsoft.com/kb/909133
"Spike" wrote in message
...
I just received a new computer with Vista and am using Outlook 2003. This
is my third computer and on the others I had this problem infrequently so
it did not bother me but now I have it with every e-mail that I write.

When I have to make a change in the text the next part of the text deletes
and keeps deleting as I am typing in another letter or word or making a
change. This is most annoying as I find the only way to combat this is to
cut and paste and it is very time consuming. How can I fix this quirk in
the program so that I can make changes without having to cut and paste?
Thank you for any help that you can offer!

Marilyn!