Hi,
Look for the file \boot\bootfix.bin. If this file is present then WinPE will display the message Press any key to boot WinPE (this is not the exact text). If you do not press a key then the system will boot from the next device specified in the BIOS (boot order).
If this file is not present then copy it to the boot directory. The AIK or OPK will have this file.
So I would copy this file to your Flash Disk and then make sure that the BIOS is set to boot next to the disk drive (or after the CD). The only possible issue is having to watch the boot screen when you are booting your system to watch for the Press any key message when booting the first time from your Flash Disk.