"Specified file failed upload test" when using ShopperPress

I have just taken over a Shopperpress theme website which previously ran on linux, but we are now running it on a Windows IIS6 server.

The site works perfectly and orders can be placed, however we can’t upload any images. The following error appears every time “Specified file failed upload test”.

I have checked the settings in the table wp_options, and “imagestorage_path” is set to the following:
C:\Inetpub\vhosts\earguards.co.uk\httpdocs/wp-content/themes/shopperpress/thumbs/

The “Quick Help” section verifies this as being correct, and all permissions are also correctly set as well.

I have approx 50 standard wordpress sites running on the server without any issues. I have already contacted the developers PremiumPress but they told me it was a Windows issue and to try here (bit disappointed with their response to be honest)

Any ideas?

Cheers, Chris.