If you upgrade to the OS 4.5 and you use a different carrier OS such as Orange or Vodaphone and yada yada, you muc remove the vendor.xml file. So where is it?
C:\Program Files\Common Files\Research In Motion\AppLoader
go there and find the vendor.html file and then delete it. After that you can open the application loader and it will show your new OS to upload to your blackberry.
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