Microsoft recommends:
Minimum Configuration (to play 720p video)
Windows XP
Windows Media Player 9 Series
2.4 GHz processor or equivalent
384 MB of RAM
64 MB video card
DVD drive
1024 x 768 screen resolution
16-bit sound card
Speakers
Optimum Configuration (to play 1080p video with 5.1 surround sound)
Windows XP
Windows Media Player 9 Series
3.0 GHz processor or equivalent
512 MB of RAM
128 MB video card
1920 x 1440 screen resolution
DirectX 9.0
DVD drive
24-bit 96 kHz multichannel sound card
5.1 surround sound speaker system