I cannot run my accounting software on windows 8. its says cannot run exe file.

Method 1.

Temporarily disable security software, reboot the computer and check if you are able to install the software.

Disclaimer: If you have to temporarily disable the antivirus to install other software; you should enable the antivirus as soon as you're done. If you're connected to the Internet or a network while your antivirus software is disabled, your computer is vulnerable to virus attacks.

Method 2.
You may boot the computer in a clean boot state and check if this helps.

When you start Microsoft Windows, typically there are several programs that start automatically and run in the background that may interfere with the installation. These programs may include antivirus and system utility programs. When you perform a clean startup procedure, you prevent these programs from starting automatically.