There are many paid password cracking softwares available, but I wouldn't recommend one.
Here's a simple way to reset forgotten Windows 8 password using Windows installation disk only:
1. Boot from the Windows 8 installation disc (or Windows 8 Recovery Drive).
2. From the Windows Setup menu, click "Next".
3. Select "Repair your computer"
4. Under Choose and option, click on "Troubleshoot".
5. Under Advanced options, click "Command Prompt".
6. At the command prompt, run the following commands:
ren Utilman.exe Utilman.exe.bak
copy cmd.exe Utilman.exe
7. Close the command prompt and then click "Continue".
8. The computer should now boot and present the logon screen. Here click Windows Key + U.
9. At the prompt you can now change the password, by typing the command:
net user administrator 123
This will set the password for the Administrator user to be 123.
10. Closing the command prompt, you should now be able to log back onto Windows 8 using the password you have provided in the last step