This is a common problem with recording multi-track sounds on a computer. It is caused by something that is known as 'latency' - basically it is down to the fact that the signal from your microphone takes a different route through the system to that taken by the audio playback. The AT2020 is a USB microphone and the signal therefore has to pass through the USB bus circuit of your computer. Despite claims of multi-tasking your computer cannot do everything at once so it naturally creates delays to make itself work properly. This will be more of a problem with a computer which has a 'cut-down' processor such as a Celeron (Intel) or Duron/Sempron (AMD). These processors are really only intended for basic computing (office type) use and are not really up to the high demands of video or audio recordings.
You may get some improvement by tweaking some of the audio settings for your sound-card to change the hardware acceleration.
This article may help you http://www.pcmus.com/latency.htm
If you are using a dedicated midi /music recording program such as Cubase then you may be able to correct the time lag using the 'quantise' function after recording your vocals. However the best solution is likely to be a hardware one Either buy a proper multi-track mixer/recorder designed for the purpose or get a second USB microphone - play your backing track on an external device - using one microphone to capture your voice and one to re-record the backing track (the only downside for this second option will be a gradual degrading of the sound quality each time it is re-cycled)