Windows XP is just another name for Windows NT
Let me explain, the Windows OS was originally based on DOS. Here is a list of Windows OS based on DOS in order:
1. Windows 1.0, year 1985
2. Windows 2.0, year 1987
3. Windows 3.0, year 1990
4. Windows 95, year 1995
5. Windows 98, year 1998
6. Windows ME, year 2000
After Windows 3.0 a new Windows OS was made Windows NT. The first NT version was called NT 3.1 even though it was the first version, because Microsoft wanted NT to look like a continuation of Windows 3.0
Windows NT OS in order:
1. Windows NT 3.1, year 1993
2. Windows NT 3.5, year 1994
3. Windows NT 3.51, year 1995
4. Windows NT 4.0, year 1996
5. Windows NT 2000 or NT 5.0, year 2000
6. Windows XP or NT 5.1, year 2001
7. Windows Vista or NT 6.0, year 2007
8. Windows "Vienna" (Windows 7) or NT 7.0, year (expected) 2009