XviD avi files (some properties attached for one of them.) Hardware is dual core amd 3800+ processor, with 2GB RAM, nvidia geforce 210 graphics card with 512MB RAM. 2x500GB hard drives running windows 7 pro. About 6 years old. I built it as a media centre PC - as mentioned above originally running XP. Pretty sure its just the video format - and they definitely ran in 1.2.3 - several were episodes we'd already watched. We did have some issues with earlier MP (0.23) - had to watch some episodes in vlc - but none with 1.2.3.