Starting with version 4.02, Sandboxie fully supports Windows 8 without qualifications on both 32-bit and 64-bit editions.
Please visit the Download Sandboxie web page.
With version 3.76 and earlier, Windows warns that Sandboxie v3 is not compatible with Windows 8. This warning applies to versions of Sandboxie before 3.72. When using Sandboxie version 3.74 or later, you can safely disregard the warning message from Windows 8.