maybe this will help. or maybe this is what you guys tried. I got this working on my windows 7 x64 and xp machines.
http://androidforums.com/all-things-root-evo-4g/91831-www-unrevoked-com-went-live-4.html
follow those instructions exactly as the first post tells you to do. But there are steps missing. After you'd finished those instructions, boot your phone into the bootloader and go to Recovery. You should hear the USB connect sound from your PC again, and the PC will ask you to install drivers AGAIN. point windows to that unzipped folder.
Next, reboot your phone all the way to the lock screen. Again, windows will ask for drivers. Point it to the unzipped folder AGAIN.
Basically, there are 3 USB modes your phone can be in. Windows sees each mode as a separate device; that's why you need to install those drivers 3x. Once you do that, you should be all set, and unrevoked will recognize your phone.
I spent hours figuring this out when I first tried to root (with unrevoked). And then shortly after that, I realized unrevoked was not a full jailbreak, so I simplerooted
however, the effort you spend to get the drivers installed is not wasted. You need them for doing adb work as well.
-edit-
getting the drivers installed on your PC has nothing to do with your android version. the drivers work for both. As for rooting with unrevoked, yeah if you're already on stock 2.2, you can't root.