Due to huge demand, the Official ET:QW site is currently down, and not showing any mirrors, however they have released the official link to a torrent of the demo, found below.

Requirements

SOFTWARE REQUIREMENTS * Microsoft(R) Windows(R) XP/Vista (Windows 95/98/ME/2000 are unsupported)
* Microsoft DirectX(R) 9.0c (included)

HARDWARE REQUIREMENTS * CPU: Intel(R) Pentium(R) 4 2.8 GHz or AMD(R) Athlon(TM) XP 2800+ processor (Pentium 4 3.0GHz or equivalent for Windows Vista)
* RAM: 512MB RAM (768MB for Windows Vista)
* Video Card: 128MB NVIDIA(R) Geforce(TM) 5700 or ATI(TM) Radeon(TM) 9700 (full list of supported chipsets below)
* Sound Card: 100% DirectX 9.0c compatible sound card
* HDD Space: 750MB of free hard drive space, plus 200MB for Windows swap file
* Internet: Broadband connection and service required for full gameplay and downloads. Features may change without notice.

SUPPORTED VIDEO CHIPSETS NVIDIA * GeforceFX Series (5700 or higher, not including 5700LE and 5700VE)
* NVIDIA Geforce 6 Series (6200 or higher)
* NVIDIA Geforce 7 Series (7300 or higher)
* NVIDIA Geforce 8 Series
* ATI Radeon 9700 or higher
* ATI Radeon X700 or higher
* ATI Radeon X1650 or higher

Download Torrent
Official Site
Official List of Mirros