What happens if you download the installer to the desktop and run it from there? See if reinstalling fixes whatever problems you are having, if it doesn't, then at least it should have put the proper files in the right places for uninstalling.
I'd suggest backing up your itunes library to a cd before going too far trying to correct this problem.I lost a number of purchased tunes with the itunes 7 debacle.BTW itunes 7.0.1 seems to be ok so far.