Test Your Settings
Whatever BIOS your PC may use, it's important to make sure all of its USB options are activated. These may include "USB Legacy Support" and "USB 2.0 Controller". Whenever possible, you should also extend the setting that governs the timeout period for how long the BIOS will wait for a response from the USB flash drive. Some of these devices take a half minute or more to respond to a boot-up instruction from the motherboard. Thus, if you see a BIOS option that reads something like "USB Mass Storage Reset Delay", set that value as high as it will go whenever boot-up problems should occur.
Many motherboards offer options to change the emulation type for certain devices, including USB flash drives. The default setting for "Emulation Type" is usually "Auto". If this causes problems, try other settings instead. First, try "Hard Disk", then try "Forced FDD", then try "Floppy" as your last option. For each new setting for emulation type, your computer must reboot for it to take effect. Also, you must reset the boot device sequence or priority in the BIOS each time as well.
Many older USB flash drives, especially those that belong to the first generation of USB 2.0 memory devices, have problems with data transfer between the flash drive controller and the BIOS. If boot-up won't work with the flash drive set to "High Speed Mode," try the slower "Full Speed Mode" option instead (if the BIOS offers that choice). This slows the data transfer rate from a maximum of 60 MB/s to less than 1.5 MB/s, which makes Windows boot up at a terribly leisurely pace. Your next step should be to make a BIOS update, so that the PC will also boot using the far faster "High Speed Mode."