Make it in Vegas, just put 5 text events on the timeline number them 5-1 and let them go for a second each. Render to an avi file and there ya go. For an added touch you can find a little beep sound effect to add when the numbers change, you can use custom backgrounds or graphics, whatever you want.
There is an effect already loaded in vegas for that.
Open a new veggie
insert an empy event
Click on:
-video even fx
-time code
-then modify to the type of time you want to show (probably seconds)
Render the clip
Insert the clip and set its velocity to -100
Use cookie cutter so that just the time section is showing
BAM, your countdown.
If you render in a format that will keep the alpha opacity of the background you won't have to use cookie cutter, but I'm not sure if that is possible.