When doing stsadm restore, sometimes you might end up getting access denied error.
If you get this,
first, check whether your command prompt is running as Administrator
even if that does not solve the issue, go and add your Administrator account (assuming that is your main services account), as your primary site collection administrator (this is in Central Admin).
Make sure Secondary Admin is not the same as the primary admin. Secondary admin ideally can be the machine's administrator (assuming your server is a member server of the doman and you alreay have added the domain administrator as the primary admin)