Link: http://windowsxp.mvps.org/aupolicy.htm

When you open the Automatic Updates tab in My Computer Property sheet, or from Control Panel, all of the Automatic Updates configuration options may be grayed out. This happens due to any of the following reasons:
You're not logged on as Administrator (or equivalent)
Automatic Updates Policy is enabled
Automatic Updates (and Windows Update) access is blocked via Group Policy
Resolution
To make the Automatic Updates options configurable by the user (only for stand-alone systems), remove the restrictions 2 & 3 above.
Click Start, Run and type REGEDIT.EXE
Navigate to this location:
HKEY_LOCAL_MACHINE \ SOFTWARE \ Policies \ Microsoft \ Windows \ WindowsUpdate \ AU
In the right-pane, delete the two values AUOptions and NoAutoUpdate
Navigate to this location:
HKEY_CURRENT_USER \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Policies \ WindowsUpdate
In the right-pane, delete the value DisableWindowsUpdateAccess
Using the Group Policy Editor - for Windows XP Professional
Click Start, Run and type gpedit.msc
Navigate to the following location:
=> Computer Configuration
==> Administrative Templates
===> Windows Components
====> Windows Update
In the right-pane, double-click Configure Automatic Updates and set it to Not Configured
Then, navigate to this location:
=> User Configuration
==> Administrative Templates
===> Windows Components
====> Windows Update
In the right-pane, set Remove access to all Windows Update features to Not Configured

Tags: windows