Encountered error while opening iTunes after upgrade

Hello,
I tried to upgrade iTunes in my Vista PC, and it went well. But, when I tried to open it, I got this error message (see below).
I tried reinstalling, but I still got the same error message. I then tried to remove iTunes then re-downloaded iTunes 9 and installed it again.
It was so frustrating because I got the same error message. What shall I do next?
Any advice on what to do next?

iTunes
Apple Application Support was not found.
Apple Application Support is required to run iTunes. Please uninstall iTunes, then install iTunes again.
Error 2
