Cannot Boot from USB disk

I try to install a fresh copy of win7 on my Acer laptop but cannot boot from usb disk. I did everything, boot order, legacy, bios update etc. I've even tried UEFI bootable disk. Still cannot boot from usb disk, its directly load the windows 7. What would be the problem?

Back up your files, unplug all other HD's and peripherals,

Boot into BIOS setup, reset to defaults, save changes. If an EFI BIOS you'll need to decide if you want UEFI BIOS vs. Legacy BIOS. If UEFI follow these special steps for Install. Otherwise enable Legacy BIOS or Compatibility Support Module to proceed with normal install:

Set DVD drive first to boot, HD second, set SATA controller to AHCI, Save changes and Exit.

Then boot into Windows 7 installer DVD or flash stick burned or written using tool and latest ISO for your licensed OS version here. I would use a fresh ISO written with tool in Step 2 as yours is in question.

At first installer screen Press Shift + F10, or enter System Recovery Options to open a Command Box, type:

DISKPART
LIST DISK
SELECT DISK 0 (after confirming Windows 7 target HD #)
CLEAN
CREATE PARTITION PRIMARY
FORMAT FS=NTFS LABEL="WINDOWS 7"
ACTIVE
EXIT
EXIT

Next click Install Now, then Custom install to Clean Install Windows 7 to the partition you created. If it fails then report back at which step and the verbatim error message.

In addition read over these steps to understand the tools and methods which work best to get and maintain a perfect Clean Reinstall - Factory OEM Windows 7. The steps are the same for retail.

After install you can adjust partition size or Create new partitions for data, etc. in Disk Mgmt. You are creating the partition here for the purpose of wiping the HD and marking Active which overcomes certain install failures.

Thanks for all the tips but the thing is it's not booting from USB drive anyway. Like there's no boot disk on computer. How can I install windows when I cannot boot from disk?

You must have missed this step:

Then boot into Windows 7 installer DVD or flash stick burned or written using tool and latest ISO for your licensed OS version here. I would use a fresh ISO written with tool in Step 2 as yours is in question.

Once you have a good flash stick installer, reboot and tap the Acer F12 key for BIOS Boot Menu, select Flash stick to boot under USB, Removable or HD's. You may need to expand one of those listings to see the stick.

If this fails see if you can boot the stick in another PC. If so then enter BIOS setup on the Acer to see if there are any USB settings that needs adjustment. If in doubt ask back here about a setting providing a camera snap if possible. You must Save any changes before exiting BIOS.