For Any Conversion:
For VSS Based Conversions (XP/2003):
Converter not installed/run as admin (reinstall)
Converter not installed to C:\ (reinstall)
"MS Software Shadow Copy Provider" from Service.msc (in the case of XP) is disabled or replaced (by another VSS backup software) ... remove other software and enable the service
"Volume Shadow Copy" ensure that it is enabled
Check for VSS errors in the System's Event Log (eventvwr) - troubleshoot these, and you should fix it (example: )
For stcbasic.sys Based Conversions (2000/NT4):
Converter not installed/run as Local Admin (reinstall)
Converter not directly installed to C:\ (reinstall)
chkdsk the drive for errors on a reboot