Windows 7 X64 compatibility?

Yes, Crossfire is compatible with Windows 7 64 Bit, just not with Windows 7 32 Bit.
Just install it the normal way, but ensure, that you got the latest 64 Bit Graphic and Directx Drivers installed, and that your Windows Updates are up to date.
You should run the Setup and the Gamestart as Administrator.
Also ensure, that Crossfire.exe, gameguard.des and gameguard.mon files got full access through your firewall and turn your antivirus program off for the installation.
When you followed these Steps, Crossfire should run fine