Error 450 whenever I attempt to download iTunes purchase!!!

Question

When I purchase an item on iTunes and it goes to the "downloads" folder, sometimes it will stop downloading and says that it has encountered error 450.
I'm losing money and I can never tell which songs will download and which will fail.
Can someone please tell me what the error means, how to fix it, and/or any way to download said purchases?
I've already sent a complaint to iTunes and haven't received any information.
Please help!!

Answer 1

These are the instructions I received from Apple. Even though the IP addresses were not getting blocked by parental controls I still entered them into my daughters "allowed" list and this solved the problem. I did not have to go all the way to step 3. However I did step 2 first and this did not fix the problem until I took care of the parental controls suggestion. Good luck
Step1)
Error 450 is usually associated with the Parental Controls in Windows Vista. To download purchases from the iTunes Store, you will need to add the following Internet addresses to your Parental Control's safe list:
http://www.apple.com
http://itunes.apple.com
http://ax.itunes.apple.com
If you require assistance adding these addresses to your Parental Control's safe list, please contact Microsoft or your computer's manufacturer.
After adding these addresses to your Parental Controls safe list, you should be able to download your purchases by choosing Check for Available Downloads from the Store menu in iTunes.
Step 2)
If you still are unable to download your purchases, please make sure you're using the latest version of iTunes, which is available free of charge:
http://www.itunes.com/download
Then, follow these instructions to use Network Diagnostics:
1) Open iTunes.
2) Choose Diagnostics from the Help menu at the top.
3) Select Network Diagnostics and click Run. A report will show your Network Interface Status, your Internet Connection Status, and your iTunes Store Connection Status.
Step 3)
Using Network Diagnostics helps you to locate the issue that is preventing you from downloading your purchases. If not, please respond to customer support email with the following:
- The results provided by Network Diagnostics
- The type of Internet connection you use, such as dial-up, cable modem, DSL, or corporate network
- The name of your Internet service provider (ISP)
- The name of any firewall, antivirus, or security software installed on your computer