You have accidentally created a very short Loop Region (or "selection"), and your Loop Button in the preview window is on, so the playback is repeatedly looping inside the selected region (no matter how short).

Unless you specifically want to loop your video, just click the loop button (the circular arrow) to keep it off most of the time.

If you accidentally create a loop region, just click an empty part of the timeline to de-activate it. (If the Loop Button is active, however, your playback will still loop if it goes into the region -- even if it isn't blue.)

Looping is certainly a useful feature. You might want to play an audio section over and over while you make some adjustment to it, or repreatedly play a video transition from one clip to the next while you adjust it until it's just right. Vegas is really cool that it lets you change things while they are playing, so you can see the changes immediately.

You create a Loop Region / Selection by clicking and dragging on an empty part of the timeline. You can also double-click any event on the timeline and Vegas will create a selection that encompasses just that event. The lower right corner of the Vegas window will always show you how long the selection area is. You can also place the playback cursor and press "I" (for "In"), place the cursor at a second point and press "O" (for "Out"), and create a selection region or loop region in this way also.

Of course you may have already solved your problem by now, but here's a little graphic to help those who come after you:

[IMG=http://i262.photobucket.com/albums/ii106/timlan635/LoopRegion2.png]