A handful of users have reported that Tunes uses 100% of CPU.
Fix:
This solution has been documented on Apple forums by several people.
Windows 7
1. Quit iTunes
2. Start an elevated command prompt (Start Menu -> All Programs -> Accessories -> Right Click Command Prompt -> Run as Administrator)
3. type netsh winsock reset and then press Enter
4. reboot your machine
5. restart iTunes.
Windows XP Service Pack 3 Users
Go to Microsoft support page. To reset the Winsock settings automatically, click the Fix it button or link.
Windows Vista
1.Click on Start button.
2.Type Cmd in the Start Search box.
3. right-click cmd.exe, click Run as administrator, and then press Continue.
4. Type netsh winsock reset and then press the Enter.
5. Restart the computer.