I had the same issue using Win7, here's what you do:
Log in under the administrator account, don't just choose "Run As Administrator" from your user account as it still gets stuck on Ramdisk (at least it did for me)
1. So, as stated, log in as Administrator
2. Right click Redsn0w and choose properties
3. Under Compatibility tab go to Run this program in compatibility mode for and choose WinXP SP3. Also from here on the bottom
check the box run as administrator. Click apply
4. Just in case I right clicked Redsn0w again chose "Run As Administrator"
even though I was already logged in and made above settings.
That should do it.