53 00 74 00 61 00 72 00 74 00
which is hex for the word Start. You can use any word you wish as long as it is 5 characters or less. Leave the null characters alone (00). You are editing a program. If you do not want to leave explorer unusable, you best try this with a copy. Replace all copies of 53 00 74 00 61 00 72 00 74 00 with the word of your choice.
The flag bitmap can be replaced by a bitmap of your choice if its the same size. The bitmap is embedded in USER.EXE which is found in %systemroot%\system32. Using a copy of user.exe and an icon editor, change the flag icons with your bitmap. There are at least two flag icons, sized 16x16 and 22x22 which need to be replaced. The different icons are there to support different screen resolutions.
If you are careful and use copies of the executables, you can make these changes. I have actually made these kinds of changes as an experiment. These kinds of bit twiddling were common in the days of DOS and Win3.x. But I personally wouldn't recommend it.