This issue started without apparent reason in a windows XP SP2 workstation with SAPGUI 620. File "C:\Windows\system32\drivers\etc\services" includes the line:
sapdp00 3200/tcp
and everything else seems to be OK with the file (permissions, etc.)
No obvious changes or installations have been made on the workstation that might be affecting.
I have tried login as a different user account, turning off the windows firewall, copying the services file from a working machine and re-installing SapGUI, but none of these have worked.
Thanks for your help!