It requires DirectX9.0c. Still your video card may not meet the minimum below. Check your specs and open a ticket with support if you meet them all except for XP. It's my understanding there are several people playing Rebellion on XP currently.

For Sins of a Solar Empire: Rebellion, the requirements are:

Minimum Requirements -

• Windows 8 / 7 SP1 / Vista SP2
• 2.2 GHz Processor or Equivalent
• 2 GB RAM
• 256 MB DirectX 9.0c Compatible Video Card w/Pixel Shader 3.0 Support (AMD Radeon X1650 / Nvidia* 6800 or Better)
• DirectX 9.0c Compatible Sound Card
• 5 GB Hard Drive Space
• Keyboard and Mouse
• DirectX 9.0c
• Broadband Internet Connection for Multiplayer
• * Nvidia GeForce users must have driver version 296.10 or later.

Recommended Requirements -

• Windows 8 / 7
• Intel Core i5 / i7 Processor or Equivalent
• 4 GB RAM
• 512 MB DirectX 9.0c Compatible Video Card w/Pixel Shader 3.0 Support (AMD Radeon 3x00 Series / Nvidia 8x00 Series or Better)