NOTE: These steps are outdated and no driver is required for the latest Windows 10 !!!
This installer will clean up old drivers and install a combined driver though if you’d like to try it:
https://github.com/particle-iot/windows-device-drivers/releases/tag/v6.1.0.68
Here are the steps I took to install my Spark Core or Photon USB Driver on Windows 7 / 8.1 pro when I could not find a device in Device Manager.
- Download the driver: https://s3.amazonaws.com/spark-website/Spark.zip (unzip this on your computer somewhere)
- Download Tera Term VT for windows http://logmett.com/index.php?/download/tera-term-486-freeware.html
- Plug in your Core or Photon via USB cable.
- Put your Core or Photon in Listening Mode (aka Smart Config) by holding the Mode button until the RGB LED starts blinking Blue (about 3 seconds). The Mode button (Core) or Setup button(Photon) is the one on the side with the analog pins.
- Open the Device Manager (press Windows Key + Pause Key then click Device Manager)
- Go to the Action menu and select Scan For Hardware Changes.
You should see this pop up…
Just let it fail…
You should now see the Spark Core with WiFi device listed.
Right click on it and select Update Driver Software.
Select “Browse my computer for driver software”
Select “Let me pick”
Click “Have Disk”
Click “Browse”
Browse to wherever you downloaded the spark_driver.inf file, and click Open.
- After it finds the driver, it’s not digitally signed yet so select “Install this driver software anyway”
Note: If you encounter an error at this point because you have Windows 8/8.1
You’ll need to disable your digitally signed driver enforcement policy temporarily. See this guide for the long list of wacky steps to do this: http://www.howtogeek.com/167723/how-to-disable-driver-signature-verification-on-64-bit-windows-8.1-so-that-you-can-install-unsigned-drivers/
Once you do that, start over from the Update Driver Software step. We are working on getting these drivers signed so you won’t have to do this anymore 
Bingo, bango!
Device Manager now shows:
Now you can follow this guide here to Connect your Core to your network over USB:
http://docs.spark.io/#/connect/connecting-your-core-connect-over-usb
DRIVER INSTALLED!!! NOW LET’S USE IT!
- You can try Arduino IDE’s serial monitor, Putty or I found Tera Term VT to be a GREAT alternative to both of these on windows.
What I have to do is a bit hacky, but it works.
- Close Serial Terminal Software (I’m using Tera Term VT 4.80)
- Flash Core with code that enables Serial in setup(), but waits for a character before it continues.
- Wait till Core reboots and connects to the Cloud (Breathing Cyan)
- Open Serial Terminal Software.
- Open a Serial Connection at the correct baud, 8 bits, no parity, 1 stop, no flow control.
- Press Enter.
- Bingo bango, kiss the screen! You have serial debugging over USB. Have a
or 
uint32_t counter = 0;
void setup()
{
pinMode(D7,OUTPUT); // Turn on the D7 led so we know it's time
digitalWrite(D7,HIGH); // to open the Serial Terminal.
Serial.begin(9600); // Open serial over USB.
while(!Serial.available()) // Wait here until the user presses ENTER
Spark.process(); // in the Serial Terminal. Call the BG Tasks
// while we are hanging around doing nothing.
digitalWrite(D7,LOW); // Turn off the D7 led ... your serial is serializing!
}
void loop()
{
Serial.print("Spark Core or Photon says Hello over USB! ");
Serial.println(counter++);
digitalWrite(D7,HIGH);
delay(500);
digitalWrite(D7,LOW);
delay(500);
}
I initially had a timer and state machine in the main loop, but this is way more efficient and has no effect on the user code once you are out of setup(). This is nice because it calls the background tasks while we are sitting there waiting for the terminal program to open, so you could effectively still reprogram your Spark Core while it’s in this waiting loop.
If you do use Putty, make sure you are using Serial under the Session tab
Not the Connections tab… or it will DING at you. 
I learned that the hard way… thanks to @Dave
Thanks again.