I'd also try holding down N when PT is starting, then pick your audio interface in the Playback Engine. Once the Playback engine is set, then I would go in and default your I/O settings. After doing that, open you session and make sure your track inputs and outputs are set properly.
And one last thing; make sure Windows is not using your PT audio device as it's default sound device.
If this does not help, then like the others said, post your system details so we know a bit more about what is going on. Read the
Help Us Help You for general troubleshooting and optimization tips and for how to post the proper system information.
Thanks,
Will