specified service already exists error during msi setup installtion

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.NET3.5

Windows

.NET

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Hi please help me with this problem msi setu

i have created a windows service setup c# project in visual studio 2010 and for the first time it got installed succesfully and the services which i have installed are running perfect.But when i tried to reinstall the same setup, first all required files in setup are copied into the destination folder after that iam getting >>"error 1001 the specified service already exists" and my installation is getting roll backed.
So I tried stopping and deleting the service before the reinstallation is taking place by overriding the custom actions onBeforeInstall(and onbeforeuninstall) methods and after that when i tried reinstalling, now iam getting>>"error 1001:an error occured during unistallation->the specified service doesnot exists as an installed service" and the installation is again rollbacked.
If i uninstall the old version manually and then try installing the new one at taht time its working.
But i dont want to uninstall it manually since its installed in more than 2000 people, but iwant the upgrade should be done automatically.
Plzz help me.....
Thanks in advance