Essentially what Sam and Connor suggest, approached slightly differently. Sam's method is likely cleaner. The linked video is just a rough demo...
Grab a shape from images on the Net or create your own with graphics tool and fill it with a color that does not appear in your video.
Place your video on track one and the image you created on track 2 (above track one).
Click the yellow star at the bottom-left of the track one image and use the SCALE and POSITION to place it where you want over the background.
Move the scrubber (the red line) if necessary, to see the desired portion of the background. APPLY. That becomes SEQUENCE ONE.
Click the + just above the time line and to the right of "sequence one." That will clear the time line and you will be creating sequence two.
Place your background video on track one. Then click the SEQUENCE tab (top-left) and place SEQUENCE ONE on track two (above track one)
Click the yellow star in track two, ADD EFFECT, click on the GREEN SCREEN choice, click on CONFIGURE EFFECT and, lastly, click on the effect you created.
It then should be filled with a white/black checkerboard indicating where the background scene will be shown.
On the time line, click the yellow star on track one, and use SCALE, POSITION, CROP, etc. effects to fit the desired portion of the background into the "hole" you created in the last step. APPLY.
http://hevanet.com/hb/heart.wmv
