If you have set up a KMS Host / Client system (see here) but have not reached your 25 Client threshold yet and you are over your 30 days grace period, the chances are your Windows 7 Clients are notifying you that copy is not genuine. If you plan to reach this limit very soon (within 30 days) you can always reset your client machines by using slmgr /rearm. A restart will be required but you will be on a fresh 30 day grace period.
If you do not plan on reaching the threshold for awhile then you will probably want to convert your Clients to a MAK. This works very much like VLK however it does a one time activation with Microsoft for each machine you use it on. The limit of a MAK key depends on your license agreement, should you reach it you can contact Microsoft to see if you can extend it.
To request a MAK you can either log into your MVLS and request one through your account, or you can call Microsoft Product Activation (UK: 0800 0188354), provide your enrollment number and they will process a MAK and e-mail you with it.
Once you have your MAK key simply click on Change product key under Windows Activation. This will automatically change it from KMS to MAK and connect online and activate with Microsoft. Alternatively you can use slmgr /ipk <MAK>.