Apple released iTunes 9.1, for both Windows x86 as well as Windows x64, which comes with several new features and improvements, including:

• Sync with iPad to enjoy your favorite music, movies, TV shows, books and more on the go
• Organize and sync books you've downloaded from iBooks on iPad or added to your iTunes library
• Rename, rearrange, or remove Genius Mixes
• Several Security Fixes

 The 'iTunesSetup.exe' is basically a compressed archive which contains seven different MSI's which together take care of installing everything which is needed to run iTunes:

The correct in order to install these MSI's is:

• AppleApplicationSupport.msi
  
  With iTunes Apple ships two 'support' MSI. The 'Application Support' msi has the core files for 'Application Support' within iTunes.
  
  
• AppleMobileDeviceSupport.msi
  
  With iTunes Apple ships two 'support' MSI. The 'Mobile Device Support' msi has the core files for device management of devices which can be connected to iTunes (iPhone, iPod, Ipad).
  
• Bonjour.msi
  
  "Bonjour, also known as zero-configuration networking, enables automatic discovery of computers, devices, and services on IP networks. Bonjour uses industry standard IP protocols to allow devices to automatically discover each other without the need to enter IP addresses or configure DNS servers. Specifically, Bonjour enables automatic IP address assignment without a DHCP server, name to address translation without a DNS server, and service discovery without a directory server. Bonjour is an open protocol which Apple has submitted to the IETF as part of the ongoing standards-creation process. To learn more, check out the Bonjour Protocol Specifications which detail the technologies that make up Link-Local and Wide-Area Bonjour."
  
• QuickTime.msi
  
  Configuration and installation of QuickTime is covered here.
  
• iTunes.msi
  
  the Itunes.msi contains the core files needed to run Apple iTunes.

The 'AppleSoftwareUpdate.msi' and 'MobileMe.msi' are optional and only needed if you want to use the Apple Auto Update feature:

 and/or MobileMe.