Start computer from Flash drive - How do I tell the computer to do it?

I am using Windows Seven Professional version, 32 bit on a Sony Vaio Laptop with Intel Centrino Duo.
I have Windows Defender Offline on a flash drive. Now I want to make the computer restart and boot from the flash drive so that I can run the Windows Defender Offline app from that location. I tried just inserting the flash drive into the USB port and restarting but it booted normally. I think there must be a way to tell the computer to look in the flash drive for a boot-able application. I would appreciate any help with this.

I was not able to make it work. I am trying to restart my computer and have it boot from the USB flash drive. I inserted the flash drive which has the WDO _MEDIA64 (H) app on it. I restarted the computer and pressed F8 until I saw the advanced screen. However I do not know which option to choose in order to make it go to the H: drive and read the application. I will need detailed instructions of each step in order to do this correctly.
I am trying to use the Windows Defender Offline tool to find and remove suspected malware on my computer.
Also, looking at the name of the file "WDO_MEDIA64" I think my friend may have downloaded to the flash drive the wrong version of the WDO tool, namely the 64bit instead of 32bit. My system is Windows Seven Professional 32bit (see my initial post above.)
I appreciate anyone's help with this.

To run an application you must be running a OS (Operating System).
If the USB flash drive only has the application (that you see UNDER windows on drive H: ) without a OS, you won't be able to start or boot it.

The drive letters that you see under Windows are assigned by Windows. They don't exist on boot.
When you launch the boot menu (F2, F8, F12..?) you wont see those letters. You will see the name of the device.

You will need to go into the computer's Setup screen, and set it to be able to boot from a flash drive.

Shut down the computer, then power it back on. As soon as the Sony Vaio splash screen disappears, you will need to immediately start tapping the F2 key or the Assist key. You will soon see an option for going into the Setup screen. If you can't get into Setup in this way, it may be necessary for you to remove the battery and hard drive and then try again.

Here is more information about going into the Setup screen, as well as the possibility of having to remove the battery and hard drive first:

http://community.sony.com/t5/VAIO-Up...p/60063/page/2

Once in the Setup screen, you'll go to the Advanced tab and enable External Drive Boot. Then go to the Boot tab and make sure that the USB device is earlier in the boot order than the internal hard drive.

Here is more information:

https://community.sony.com/t5/VAIO-U...tick/m-p/66951

Make sure you save the changes you made.

Exit the Setup screen, power down, and reinstall the battery and hard drive if you removed them. Then plug in the WDO flash drive, and power on. Your Vaio may boot automatically from the flash drive; but it is possible that you will see a message "Hit any key to boot from the USB drive", or something like that. If you see that message, hit a key to make sure it boots from the flash drive.

WDO takes a while to scan, so be patient when it is running.