1) Write an AviSynth Script
AviSource("your_file.avi")
ConvertFPS(25) // or 23.976 or 29.97 or nothing, if you don't want to change the framerate
AddBorders(0, x, 0, y) // only if your movie is NOT straight 4x3 or 16x9, change the values of x and y
LanczosResize(720, 576) //or 720, 480 for NTSC
2) Put your AViSynth script in a mpeg2 encoder ( paid - CCE, TMPGEnc, free - HCEncoder, QueEnc)
3) Put the result into your favorite authoring program