Found this on another network inventory website about the SP2 problem blocking the discovery. Thought this might help.
Q: Why I can't view information from a user PC with Windows XP SP2?
A: By default, the Windows Firewall blocks all incoming RPC traffic.
How will i fix it?
Windows Firewall includes a Remote Administration option that alters its configuration to allow Remote Procedure Call (RPC) and Distributed Component Object Model (DCOM) communication. Enabling Remote Administration option statically opens TCP 135 and TCP 445 to unsolicited incoming traffic. Additionally, communication over named pipes is permitted, and ports will be dynamically opened as needed by Windows services using RPC
To enable Remote Administration by default in the Domain Profile, add the following entry to the ICF.AddReg.DomainProfile section of the Windows Firewall INF file:
HKLM,"SYSTEM\\CurrentControlSet\\Services\\SharedAccess\\Parameters \\FirewallPolicy\\DomainProfile \\RemoteAdminSettings", "Enabled", 0x00010001,1
Script for enabling the remote administration
Set objFirewall = CreateObject("HNetCfg.FwMgr")
Set objPolicy = objFirewall.LocalPolicy.CurrentProfile
Set objAdminSettings = objPolicy.RemoteAdminSettings
objAdminSettings.Enabled = TRUE