I'm afraid I don't know that much about troubleshooting PC-to-Mac migration, but one thing that I'm sure would be helpful is to do a full suite of maintenance on the PC beforehand.
Things like cleaning the registry, rebuilding the Outlook database, making sure pictures and movies and such are in "My Pictures" and "My Movies" and so forth. In short, good housekeeping.
Also, be sure to turn OFF any internet connection, followed by any anti-virus or anti-malware you have running on the PC. That's likely to be the culprit.