A quick heads up: some promotional Microsoft Points have expiration dates. This is probably not new, but it is news to me! From the billing FAQ:
When do my Microsoft Points expire?
Purchased points don’t expire. Any points you get through promotions can have expiration dates. Typically, points expire within thirty days of the date you get them. To view any points in your account that have expiration dates, go to the Billing and Account Management page, sign in with your Windows Live ID, and then click Microsoft Points under Stored Credits. If you have points that are expiring in the next thirty days, you can click View expiring points to view the points and their expiration dates.
The good news at least, is that promotional points are used up first if they are about to expire:
How do I use my expiring Microsoft Points first?
Promotional points with expiration dates are automatically redeemed first before purchased points.
Microsoft’s billing site can be found at https://billing.microsoft.com if you want to check out if and when your points expire.
Thanks goes out to Zico on the OcUK forums for making me aware of this.