After idle windows 8.1 preview crashes, it's like it can't start hard drive after idle.

The more information you could provide about your system and the environment it is in the better we will be able to assist you.

When you say Windows 8.1 preview is crashing, is it crashing prior to boot or is the system starting to load and then experiencing a (Blue Screen of Death) BSOD?

If the system is having trouble booting to the operating system itself you will want to check the boot sequence in your BIOS and ensure that your hard disk drive is a primary boot option. It would also be advisable to ensure that you do not have any USB devices connected to your system as well. 

ASUS K55VM
i5-3210m
6GB 1600MHz ram /Hyundai Electronics/
Intel HD 4000 optimus Nvidia 630m 2GB
Hard drive Seagate ST9500423AS 500GB 7200rpm

Windows 7 and Windows 8.1 preview dualboot

Everything works fine, but if I leave my computer to idle stage and start using it again, it freezes.
Error did not occur in windows 8 nor in windows 7. Upgraded windows 8 to 8.1 through Windows Store.

Also that freeze happens when I open AIDA64.

looks like a driver problem, check with asus for updates after 8.1 ships

Check the 5 steps under sceneario 3:

Windows Kernel event ID 41 error "The system has rebooted without cleanly shutting down first" in Windows 8

http://support.microsoft.com/kb/2028504

Also press the CAP LOCKS key and look if the keybord light toggles to see if it is a hardware or software issue.

I can move cursor and caps lock light works, but computer is frozen, can't open anything, only way is holding power button to shut down.

so you have a software issue.

Use the clean boot approach until you find the program which causes it.

How to perform a clean boot to troubleshoot a problem in Windows 8

https://support.microsoft.com/kb/929135/en-us