I know this is an older post but for what it is worth....
I was having this same issue. I reinstalled, even downgraded to an older build. Launched the program using the "run as administrator" and tried logging into the local computer and running it as different users. I even tried to preinstall the client before the P2V process. All to no avail. In the end I ended up running the program as myself but when selecting the server I wanted to convert and giving the credentials, instead of using a domain admin account I ended up using the server's (source) administrative account. I assume any local admin account (non-domain account) would work.
SERVER: {source server name}
USERNAME: {server name}\administrator
PASSWORD: {admin password}
Hope this helps someone. I waisted a couple days on this,
Chris
VMWare vCenter Converter Standalone 5.5.3
Local OS: Windows 8.1
Source OS: Windows Server 2008 R2