What player are you using? That may be your problem. Or you could have it on the lower quality setting....

This is from the DCRP Review site:

Movie Mode

The PowerShot A570 has the same high quality movie mode as Canon's other DIGIC III based cameras. You can record video at 640 x 480 (30 fps) with sound until the file size hits 4GB. That takes about 32 minutes, which is pretty good these days. Do note that you will need a high speed memory card in order to record long continuous movie clips.

You can record longer movies by either lowering the resolution or the frame rate. The other resolutions are 320 x 240 and 160 x 120, with the latter having a 3 minute recording time limit. The frame rate can be lowered to 15 fps (and it's locked there at the lowest resolution), though I don't recommend doing this, as your video will be awfully choppy.

There's also a "Fast Frame Rate" mode available, which lets you record up to 1 minute of 320 x 240 video at a whopping 60 frames/second. This is great for videos of fast moving subjects.

As is usually the case, you cannot use the optical zoom lens while recording your movie clip. You can, however, use the digital zoom. The image stabilizer is active during filming.

Movies are saved in AVI format, using the M-JPEG codec. The A570 IS does not have any in-camera movie editing tools, unlike some other Canon cameras.