After Group Policy refreshes, it can take up to 20 minutes before client computers appear on the Computers page in the WSUS console. If you initiate detection manually, you do not have to wait 20 minutes for the client computer to contact WSUS.

At the command prompt, type wuauclt.exe /detectnow. This command-line option instructs Automatic Updates to contact the WSUS server immediately.

On Windows 2000 machine:

secedit /refreshpolicy machine_policy /enforce