Uninstall itunes and reinstall it.
When you uninstall, you have to uninstall like 5 programs to get it uninstalled all the way though, not just itunes.
Open up control panel and open add/remove programs (or program features/uninstall a program if your runnin vista or 7)
Then uninstall these things, in this order. Doing them in this order is important, for uninstalling correctly.
1.apple mobile device support
2.apple software update
3.apple application support (if its there, if not dont worry about it)
4.bonjour(if its there)
5.quicktime
6.then iTunes
Once you got it uninstalled...restart the computer ( thats important too. Seems like it wouldnt be, but it is. )
Then when your computer is back on, go to itunes.com and redownload the program and install the mother fucker. It should work fine after that.
Also, find out what version of itunes you got now. Cuz, I worked for apple tech support and I know a lot of people that would have like, itunes 6 or 7 or something...and they have to uninstall and reinstall and download the new software thats out and fuckin hate it. They just wish they had their old version of itunes back. Only problem is, apple.com doesnt allow you to download previous version of itunes from their website. So you have to go here....
http://www.oldapps.com/or more specifically
http://www.oldapps.com/itunes.phpThey got every version down to itunes 4.
Most people like itunes 8. It was itunes 9 when they switched a lot of shit up. Anyways, hope that helps.
Oh, if reinstalling itunes doesnt work...its gunna be an issue with the computer. A driver issue, or some sort of settings issue. A lot of the time it has to do with, itunes and some other audio application are clashing/bumping heads. Dont know why it happens but it does. 99 times out of 100 though, reinstalling itunes fixes the issue.
Get at me in this thread if you still got problems