Hi,
I downloaded the Kindle for PC fix today and it refused to accept my (valid) username and password. After a bit of debugging I found one of the SSL certificates was missing on my fairly new Windows7 PC. Here's how I fixed it.
1. In IE, go to
https://firs-ta-g7g.amazon.com. If you are affected by the same issue as me you will get a certificate error screen. Click the 'go to the website' link.
2. Click the red Certificate Error box in the address bar.
3. Click View Certificates.
4. Click the Certification Path tab.
5. Click VeriSign Trust Network.
6. Click View Certificate, then Install Certificate, then Next.
7. Select the 'Place all..." option, click Browse then select 'Trusted Root Certification Authorities", click OK.
8. Click Next, then Finish, then OK.
Your Kindle for PC app will now log in.
Paul