If you are using Windows CE, then \ is the root directory. It resembles a Unix / root directory. This is the only kind of Windows where you can get a simple answer to your question.
If you are using Windows NT / 2000 / XP / 2003, the closest equivalent is the section containing the NTL files; DR, NTDETECT.COM, BOOT.INI and BOOTFONT.BIN. The BIOS and MBR will find this partition by finding on which drive the boot, MBR scan, and search for the active partition are started. Microsoft calls this system partition. I'm not quite sure how the program can find which section it was. In any case, when you discover which drive letter is, say, the letter L, you can say that L: \ is the root directory. In 99% of cases, this will be the drive letter C :.
Also, if you use Windows NT / 2000 / XP / 2003, you also have a section that contains Windows system files, such as the \ Windows directory or others. Microsoft calls this boot partition. You can get the drive letter from the% SystemDrive% symbol, as someone else said. If this is the drive letter Q, you can say that Q: \ is the root of the system drive.
If you use Vista, then things get more complicated. If you installed the DVD boot, the boot partition (containing the system files) is C: and your system partition (containing the boot files) is D: if they are not the same partition, then the partition is C :. But if you found that Windows is already running, insert the DVD and run the installer under this Windows installation, then the drive letters can be almost any.
In Windows 95/98 / ME, BIOS and MBR will search for IO.SYS, COMMAND.COM, and some others in the active section. This usually gets the drive letter C: so the root partition will be C :. As always, Windows system files can be installed in the \ Windows directory or others on any partition.
Some people talk about the desktop. Of course, every registered user has a desktop. This is similar to every Unix user home directory. It is not a root directory.
Addendum: in the second paragraph, about Windows 95/98 / ME, I typed "so the root partition will be C :." That is, the letter C, a colon, a backslash, and then a period for the end of the sentence (and not part of the directory name). When viewing a page, a backslash is not displayed. But when editing this answer to add this addition, the backslash is there exactly as it should be, just like I typed it.