To solve this problem, start by exporting the logs to a zip file, open it and navigate to “Documents and Settings/All Users/Application Data/VMware/VMware vCenter Converter Standalone/logs”
Analyse the last logfile for error. You will find that Converter generated a log file: Storing at "C:\WINDOWS\Temp\vmware-temp\vmware-SYSTEM\agent-task-***.zip"
The problem is probably in the firewalls between the physical system to be converted and the ESXi server... with the needed ports opened everything should works just fine.
Otherwise, if your ESX and the physical server to P2V are on different VLANs, add a temporary Service Console to allow them to communicate. The ESX needs in fact to communicate with the VMWARE agent installed on the physical server.
This should allow proper network traffic between ESX and the server to virtualise.
It could also be a DNS issue, so make certain you can nslookup the source server from the destination server.