Hi Sombath,
Installing the Forefront client is bit different from other setups. So use the following steps to set it up.
1. Go to the folder that contains the client setup in your DVD. You should browse to this from the command prompt.
Ex - E:\English\ForefrontClientSecurity\CLIENT
2. Then run the client setup with the switch NOMOM
Ex: E:\English\ForefrontClientSecurity\CLIENT\ClientSetup.exe /NOMOM
3. Then it will install the Forefront client on your PC and give u the completion message.
4. Finally u will have to download the latest definition files for FF and install. You can go to http://microsoft.com/Security/portal and download the updates.
Then let Windows update do the rest.
Hope this helps.
Samapth