I think you are correct in assuming that the cause of your issue is due to not installing Captivate in the default C drive location.
The VT folder containing the voices should not be copied into the Adobe Captivate root folder. The normal location of all Captivate TTS voices is at the root level of the Program Files, not where you have copied them.
I would suggest you think about either decluttering your C drive to make more room and reinstalling there or else getting a bigger hard drive and migrating your system over to that. An SSD drive (if you can afford it) will make your Captivate app work a lot better than any spinning disk drive.