Here's some info that may help - picked up from other boards where similar problems reported.
First check to see if Quicktime works normally, iTunes will not work without it.
If Quicktime is OK, then use these troubleshooting articles: XP version http://support.apple.com/kb/TS1421
Vista version http://support.apple.com/kb/TS1717
Have you installed any of the following:
K-Lite mega codecs package, QT alternative, Storm codec, WinAVI video
converter, Mpcstar, (possibly starcodec pack, 3ivx)
If so uninstall them, they are know to cause problems with Quickitme. Then download the version of Quickime without iTunes: http://www.apple.com/quicktime/download/win.html
Now Remove Quicktime using the method for Quicktime in this article (just Quicktime): XP http://support.apple.com/kb/HT1925
When you come to your C:\Windows\system32\ folder you can delete any
file or folder with Quicktime in the name. Restart you computer Install
the stand alone Quicktime and check if it starts. If it does you are in
a position to find out if iTunes is OK.
If Quicktime is OK, do you get any error message when you try to start
iTunes? If not, open your task manager after trying to start iTunes. Do
you see iTunes.exe running on the processes tab. If there is no error
and you see iTunes.exe running then iTunes is hanging and we need to
look at a slightly different route.
Sometimes it is just a problem with the itunes database file when it won't open after an upgrade. Press and hold
the shift key while trying to start itunes. Does it give you an option to choose or create a new library? http://docs.info.apple.com/article.html?path=iTunesWin/8.0/en/15502.html
Bonjour is a common cause of iTunes hanging at start up. i.e. silent
launch failure with iTunes.exe running on the processes tab. You can
uninstall bonjour if you don't need it. However it is required for
sharing libraries and other things
You could also try uninstalling it, then downloading bonjour from Apple and installing it directly. http://support.apple.com/downloads/Bonjour_for_Windows