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Free download remote control software.
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SpiceWorks

Cool network management software - to enable discovery for XP sp2 PC:
netsh firewall set service remoteadmin enable
should get "ok" as a response
If you still can not discover the machine:

1. Check WMI configuration. It easily can be done by msinfo32.exe utility, for remote access to WMI run:
   >msinfo32.exe /computer machinename for example:
   >msinfo32.exe /computer sir01
2. If you can not connect, you need to check DNS PTR records (pointers) for the machine, if the PTR wrong delete it, and on the machine run: >ipconfig /registerdns It should resubmit the machine to DNS. (check DNS A records too)
3. To troubleshoot WMI see MS article http://support.microsoft.com/default.aspx?scid=kb;en-us;875605