Technical question - Screencaps from DVD

My main computer's graphics hardware (HP Pavilion PC running XP with 1.8 Ghz Athlon processor and S3 Graphics Pro3 DDR on the motherboard) doesn't seem to support screencaps from DVD or DirectX files. Neither does the other PC (400 Mhz Celeron, Windows 98 with ATI graphics on the motherboard). Currently I'm playing them using Windows Media Player on the XP box, some sort of DVD player program on the 98 machine. On both machines DVDs won't play if I tell the software I have hardware acceleration, but things play all right without it.

Is there any software, preferably free or cheap, that can get screencaps without supporting hardware, or should I be looking at getting another graphics card? If so, what?

Thanks!