I'm running some of those dongles on a Win7 64bit machine.
Here are a few things that may help you.
When you run zadig, you should run it in 'administrator' mode.
When you start SDRSharp, you should select 'RTL-SDR / USB' from the drop down box.
Go to Control Panel, Device Manager
Under Universal Serial Bus Devices ..........
Look for 'Bulk-In, (Interface 0)
Make sure it shows the libusbx.org driver.
If so, you should be good to go.
If not, then the driver isn't installed.
If you are saying that you can't see "RTL-SDR / USB" in the drop down box ..
Then edit the 'SDRSharp.exe.Config' file and look for this line (between the quotes) near the bottom. (SDRSharp must NOT be running when you do that.)
"<add key="RTL-SDR / USB" value="SDRSharp.RTLSDR.RtlSdrIO,SDRSharp.RTLSDR" />"
Rich