Hi,
Please check the features of endpoint that you are trying to install on the Laptop's having XP operating system. Try installing only Antivirus and Antispyware first then install the PTP component and then the NTP. This will help us in narrowing on the exact reason which is causing the error.
It would be great if you could give us the following information as well.
>>Whether the clients are Managed or Unmanaged
>>The version of SEPM and SEP