Problem:


Itunes won't open itself. Ths problem might be caused by a faulty configuration file, plugins, quicktime or other programs.

Fixes:


Remove config files


You might try to delete the config files 'SC Info.sidb' and/or 'SC Info.sidd'.
To do so go to
'C:\Documents and Settings\All Users\Application Data\Apple Computer\iTunes\SC Info' (WinXP)
or
'C:\ProgramData\Apple Computer\iTunes\SC Info' (Win7/Win8)
You might have to change your folder options (advanced settings) to 'Show hidden files and folders' to make the files visible.

iTunes safe mode / Disable plugins


It might be that an outdated plugin is holding iTunes back. It is possible to start iTunes in a 'safe mode', where plugins are disabled. To do so, Start iTunes, and hold down shift, ctrl and the second shift button at the same time. (Windows)
If you use iTunes on MacOS, use the option and command keys.
If it works you will see 'plugins disabled'.

Reinstall Quicktime


Go to windows control panel -> add/remove programs -> select Quicktime -> select change -> reinstall or repair Quicktime.

Multiuser PC Itunes - User rights


Right click Itunes and select 'Runs as', select different user.

Virus scanner blocking Itunes



Your virus scanner might block Itunes access to hardware. Check virus scanner log for Itunes.

Firewall blocking Itunes


Your Firewall might block Itunes access to the internet. Check firewall log for Itunes.

Reinstall Itunes


Go to windows control panel -> add/remove programs -> select Itunes -> select change -> reinstall or repair Itunes

Corrupted Itunes library


Create a new itunes library file.