We will try to help you.
This issue is there for many customers after the launching of new version of iTunes.
Lets try reinstalling with an old version of iTunes.
The issue is from Apple, so They will be releasing a new version soon
For now lets try installing iTunes 8.2
Download and install the downloaded iTunes and try to purchace.
Let me know how it goes.
Feel free to ask anything. Click ACCEPT button to close this session.