Hey there! Stopping an instance does save on the 750 free hours of free usage. An instance in the stopped state isn't running, so can't be RDPed into. A running instance on the other hand, is using those hours even if it isn't being RDPed into. As long as the Windows instance in question has an EBS volume, stopping the instance will not remove any software, configurations, etc. It's basically like a computer that is turned off.
Elastic IPs that are attached to the instance will stay on the instance, even when stopped. It will not be removed from the instance until you explicitly remove it, or the instance is terminated. (Even then the IP remains in your account to be reused until you explicitly release it.)
Thank you to both members for the great help, and for clarifying the effects of a 'stopped' server , also for the link related to stopping an instance and confirming when retaining an Elastic IP in that a charge applies while an instance is stopped
Read through the below doc for the official information & wording. The public IPv4 address that Amazon EC2 automatically assigned to the instance on launch or start.
Since you ve chosen Elastic IP, Note that when the instance is stopped, we start charging you for the associated Elastic IP addresses.
Link- https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Stop_Start.html#what-happens-stop -- 
Thanks Jack for your additional commented information about EBS storage being billable while an Instance is stopped.
I just wanted to say that using the AWS Virtual Machine service is really showing itself to be a great product, and very convenient. The rePost forum and it's members have also been really helpful, thank you all.
I am at present using my EC2 Instance within the 'Free Tier' package, and definitely see the value of a paid package.
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Just to add to this, you save on the instance costs, but the EBS volume will still be billable.