Whenever I am click on preview it adds on the download tab but in just a few seconds there appears a red sign "download *****". On the other hand when I download the card which I have purchased it downloads just fine. I guess this means if I first make a purchase then download, it would download just fine but you don't expect me to puchase without seeing the preview first do you? I damn first need to see the preview first! Help..

Well its a technical issue. The preview system work on the basis of using p2p technology while the main downloads come strait from the iStripper server.
The best option would be to see if p2p trafic is being blocked somewhere in your network or even by your ISP. But if theres nothing you can do about that you could try to turn off the p2p option.
Go to your settings window and check the "Advanced Settings" check box at the top left of the settings screen. Then scroll down to the "Downloads" section and uncheck "Allow file sharing (P2P)"

Im not sure if a restart of the program is really required for the setting to come into effect but i would do it anyway. Also note turning off this option can make the downloads a bit slower so be patient.