When I am Downloading Previews, is there a permanent file being downloaded which is taking up disk space on my PC?

If so,where is it being stored; and how can I delete it ?

is there a permanent file being downloaded

Yes, the Preview Files are Stored Locally on your PC

If so,where is it being stored

Go to the top right by your user name, and click on the down arrow.
then select Settings
Put a check in the box Advance Settings
Now Scroll Down to the DATA section
Look at the Storage Folder PATH
that is where all of the Models files are stored.

C:\Users\USERNAME\AppData\Local\vghd\models\

use a windows Explorer folder and navigate to that Path.
( this is normally in a Hidden User Folder)
( you might have to google: How to show Hidden Folders )
Each Card has it's own sub-Folder
Both Purchased cards, and Preview cards are ALL in this same folder
make sure you are finding the Preview card number, and not one of your Purchased card numbers.
find the folder name for the Preview of the card and you can delete it.

Each Card has a card number
Each type of card has a Prefix Letter

in General:
iStripper cards start with the Letter 'e'
iStripperXXX cards start with the Letter 'f'

Classic iStripper cards start with the letter 'a'
Classic iStripperXXX cards start with the letter 'c'

VirtuaGirl Classic cards start with the letter 'd'

hope this helps

For the More Advanced User
If you have Many Previews and want to Automate the Process
you can use a Script to do this
Here is an Example:

copy it and save it to the Root of your Models folder.
Run it from there.

This is in DEMO mode, so as it is written it doesn't delete any files
it just makes a log of What it Would delete when you take it out of DEMO mode.
I leave that part up to you.
Like I said, this is for Advanced Users.

rem find all the Preview Clips and delete them if not part of a full card
@echo off
setlocal enabledelayedexpansion
rem create log if it does not exist
If Not exist RemovePreviewClips.log @echo off > RemovePreviewClips.log
rem Log the start time
Echo Started at %date% - %Time% >> RemovePreviewClips.log
rem search all of the possible card folders
For /D %%d in (a0???, a1????, c0???, d0???, e0???, e1???, e2??? f0???, f1???, f2???) do (
Echo checking in folder %%d
rem if the folder does not have any .vghd files, it is a preview card
@if not Exist %%d\%%d*.vghd (
Echo removing the previews from folder %%d
rem make a log of the clips
echo removing the preview clips from folder %%d >> RemovePreviewClips.log
dir %%d\*.demo >> RemovePreviewClips.log
rem delete the clips
rem
rem in demo mode by default
rem ***************************
rem delete the rem from the line below to allow file deletion
rem del %%d\*.demo
rem ****************************
rem
)
rem loop through all the folders
)
rem done record the time to the log
Echo Finished at %date% - %Time% >> RemovePreviewClips.log
echo Done.
Pause
rem exit

To remove all demo files at once, you can use this tool:

https://www.isworkshop.eu/index.php?p=remover

Manual:
https://www.isworkshop.eu/index.php?p=remover_what