SEP client reflecting in SEPM console

Hi

We have 5000 clients in the network and around 1000 clients are on Windows XP.

We followed the below mentioned steps to upgrade the Operating system of Windows XP to Windows 7.

1. Format the System

2. Install the image of Operating system i.e Windows 7 with SEP client installed on it.

3. But after doing this the client is not reflecting in console.

4. When we remove the hardware ID the client reflects in the console

Let me know how can we resolve the same or the best possible way for smooth OS upgradation

Regards