Hey there!
It sounds like you are having trouble updating iTunes, even though you are connected to the internet. I would start with the troubleshooting in the following article to help resolve any software conflicts, or security software issues:
Resolve issues between iTunes and security software
http://support.apple.com/kb/ts3125
If this does not resolve the issue, I would next remove the iTunes software (this will not compromise your iTunes library):
Removing and reinstalling iTunes and other software components for Windows Vista, Windows 7, or Windows 8
http://support.apple.com/kb/ht1923
Or
Removing and Reinstalling iTunes and other software components for Windows XP
http://support.apple.com/kb/ht1925
Then download iTunes with this link and install it:
http://www.apple.com/itunes/download/
Thank you for using Apple Support Communities.
Cheers,
Sterling