How can I enable it again in win xp

This involves editing the regitry, if not comfortable doing that, see if you would be able to find someone who doesn't mind and you can trust

Enable / Disable Task Manager in Windows XP Home / Pro (http://www.windowsnetworking.com/kbase/WindowsTips/WindowsXP/UserTips/Customization/EnableDisableTaskManagerinWindowsXPHomePro.html)

Hope this helps out