I just upgraded from windows 8.1 to windows 10 and reinstalled this again after not using it for ages due to lack of updates
and it says I need to renew my key but my key had updates until march 24th 2014
which according to the latest news my key should be able to get the new update

Yes it should and it does (unless you mean the prompt came from the Winstep Update Manager).

Notice that it says in the About tab of Preferences that you need to renew your license key to be able to run further updates (which normally would be true since it was only valid until May 2014) but if your license key wasn't working with it it would say 'Winstep Xtreme is shareware blah blah...' instead.

So don't worry, v15.7 is registered on your system. This said, to install the next update (whenever that is) you will indeed need to renew your updates subscription.

And since you are now running Windows 10, how about some feedback on how Xtreme is behaving with it on this thread HERE? I would really appreciate it.