If you have a WAV file of the song, you can open it in Audacity. With Audacity, you can slow it down by changing the tempo (more or less without changing pitch)and you can use a low pass filter to attenuate frequencies above the bass - which makes the bass line stand out. (Try 400 hz cutoff and 12 dB roll off.) Both of these are found in the Effects drop down.
You can also let you tuner listen to the song by plugging it into the headphones jack on you computer, if have a suitable connecting cord. Sometimes this helps, sometimes not. Ultimately, your ears will tell you when you've found the right note.
