Itunes will not run unknown Error (-50) help needed......

First check to see if Quicktime works normally. iTunes cannot work if Quicktime doesn't.
If Quicktime doesn't work, report back with any error message and a description of what happened.

If Quicktime is OK, work systematically through this troubleshooting articel. I can't tell from your post if you have already done this.
http://support.apple.com/kb/TS1717

If you can't get iTunes to work, report back with the results of the various steps, especially if the fault was user specific or system wide.

OK, so iTunes opens OK in another account so it is a user specific problem.
Just to be absolutely clear, have you carried out the all the user specific steps:
Remobe plugins
Remove preference files
Recreate Library
Check for Content files with issues.
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One of those steps usually solves user specific issues.
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Something else that you could try is to create a new iTunes library elsewhere on your disk.

Hold down the shift key and start iTunes. Keep holding the shift key until you are prompted to choose or create a library. Select "Create" and create a new library somewhere way from the your music folder eg in Documents - so it is completely separate from your existing iTunes.

Does iTunes Open like this?

If so go do Edit>>Preferences>>Advanced and Un-check "Copy to iTunes Music folder when adding to library". This is to stop iTunes copying your music files when you add to you new library.
Now try adding music from your original iTunes Music folder in stages as in the "Content files with issues" section.
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This approach has worked for someone else although they did not say if they stuck with their test library or if they ever found the problem with the original one.

That's odd, there don't seem to be any other user specific things I know of to try.

I don't think you ever told me if your Quicktime works normally, although I would expect a quicktime problem to be system wide.

What happens if you go to c:\program files\iTunes and try opening iTunes by double clicking on iTunes.exe or a shift key start?

Sometime ago I did encounter an iTunes Launch failure that was caused by a corrupt user profile, but in the end the OP's IT support identified that as a problem. I don't think we want to go there yet.

OK, I wondered if you might have bad short cuts, but if you can't start directly from iTunes.exe, it can't be that.

I wonder if you have software that loads on startup in your current user that upsets iTunes, and it doesn't load for the other user.

You could try MSConfig, to do a selective start up.
http://support.apple.com/kb/HT2292?viewlocale=en_US

If that doesn't reveal anything, I am afraid I am out of ideas.
I wonder if anyone else has any.

The only thing I have left is the corrupt user profile, but the simple way out of that is to use the other account. I don't know if that is an acceptable work around. You can move your iTunes folder from Music to the shared Music folder and then use the shift key start to access it from the other account. You might have to adjust permissions if the other account is limited user. The account needs to have write access to the itunes folder.