Problem with using my Paypal account on Xbox Live

[DK]

I've tried asking elsewhere on this site for help, but I've had no luck and no (serious) replies. I tried buying Microsoft Points with my Paypal account, and I've failed. I tried buying those Points while using my (brother's) Xbox and also while using my computer. On the Xbox, I keep getting a message telling me that my Paypal account is "unauthorised" or something along those lines. If I try using my computer, when I put in my Paypal details I get sent back to the start of the process of buying Microsoft Points again. I've used my Paypal account before on Xbox Live to buy points and an Xbox Live Gold membership, and it was successful.

Can anyone help me here?