The PC Spec from Microsoft's web page to play 1080P on PC is:
Optimum Configuration to play 1080p video with 5.1 surround sound --

• Windows XP
• Windows Media Player 9 Series
• DirectX 9.0
• 3.0 GHz processor or equivalent
• 512 MB of RAM
• 128 MB video card
• DVD drive
• 1920 x 1440 screen resolution
• 24-bit 96 kHz multichannel sound card
• 5.1 surround sound speaker system
• An AGP4x-based NVIDIA or ATI video adapter card with the most recent OEM driver updates is also recommended.

Websites to visit for more information:

• Microsoft - Choosing the Right PC for Windows Media High Definition Playback
• Microsoft - Windows Media Video 9 Series Audio and Video Codecs

Microsoft has HD Content "showcase" files to download and test  -- click here