May be we both live in a different world of sorts. Are there others like you who seem to be facing issues with instant video recently because for me it works quite well. Also , how long back did you try it ?
No matter where you are, Amazon won't stream high quality (or even HD) streams to a PC. Their streaming quality is quite poor compared to their competitors.
Don't spread FUD. I get 1080p streams on my PC when I rent or purchase videos through Amazon VOD or watch the free ones through Prime. If your PC isn't getting HD (but for some reason your other devices are), the problem is probably your computer or its network connection.
"HD playback is not available on all devices. For example, some Roku and TiVo devices don't support HD content. The Kindle Fire HD is HD compatible, along with Windows computers that meet the content protection (HDCP) requirements."
My computer is perfectly capable of playing 1080p content (I do it all the time) they just choose not to provide it to me on my non-HDCP monitor. That's clearly their problem, not mine.
I have never been able to get HD in a browser on my PC with "free" Instant Prime. I cancelled Prime because of all the obscure restrictions on streaming Instant Prime. What are you doing to get HD from Instant Prime?