Athlon X2 Dual core may be enough, but the 2.1 Ghz is a bit slow for HD video.
Does the Video have CUDA (Nvidia) or ATI Stream processing? Is the Video RAM shared? if so your 3 GB of RAM may not be enough.
Nothing puts as much strain on a Computer as Video Editing.
You might test your setup by trying the Sample Clips.
Just add two or three of the Nature.mpg on the timeline is see what happens. The Sample Clips are not heavy duty and don't put much strain on the computer. So if they produce and play well, you are trying to do too much on that computer.
Carl312: Windows 10 64-bit 8 GB RAM,AMD Phenom II X4 965 3.4 GHz,ATI Radeon HD 5770 1GB,240GB SSD,two 1TB HDs.