If the machine you want to convert is a physical one, you can't use Converter for a cold migration (not supported anymore). You can do the hot conversion, and usually it works just fine.
You can also use Symantec Ghost, for example, to create an offline image of your machine and then convert it to vSphere. Check the manual, page 20, for supported source types: https://www.vmware.com/pdf/convsa_55_guide.pdf
Good luck with your conversion, let us know if it worked.
Marcelo