Windows Mail error

Hi All,
Hope I'm in the right section.
For the last week I've been getting this error at times when I try to open Windows Mail >>> "Windows Mail could not be started because there is another instance of it is still running on your computer."
Each time I am closing out of windows mail correctly, but when I try to reopen it at a later time I receive this message. I have tried going into Task Manager and closing W.M down from there but that doesn't work either. I have to restart my computer each time which is becoming an annoying habit.
Any help would be appreciated.

Lets try several ideas and see what works.

Assumption, mail is being held open by another program.
Solution, try clean boot.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7

Assumption: Held open by a driver
Solution, try safe mode

Assumption: Defective account
Solution: Open a new user account and test mail

Assumption: Defective Windows Mail
Solution: Turn off and download a new mail service, Thunderbird, etc. At least for test purposes.