Just before you enter command prompt screen note the drive letter Windows is on:
Following instructions are based on the above example (Windows on drive "D"). If in your case you see different letter you'll have to adjust my instructions.
When you enter command prompt you should see X:\SOURCES>...
Now you'll have to find your USB flash drive.
There is no other way but to use hit or miss way.
At You should see X:\SOURCES>...
If the command prompt will change to C:\ type:dir
Do you see winload.exe there?
If not try next letter (we'll skip "d" since we know it's your hard drive).
Do you see winload.exe?
...and so on until you find your USB flash drive.
Let's say you found your USB flash drive as drive "e".
In that case type the following command at any command prompt:copy e:\winload.exe c:\windows\system32\winload.exe (<--- watch for two "spaces")