For example: Windows XP 5.1 Build 2600.5512 (SP 3)
Windows Major version is 5, the minor version is 1 and the SP number is 3
= 513
If you want to check for SP1 or SP2, use 511 or 512.
Windows 7 6.1 Build 7600 (Service Pack 1: 6.1 Build 7601)
Windows major version is 6, minor version is 1 and SP number is 1
= 611
Here is the link with all versions:
https://en.wikipedia.org/wiki/Windows_NT
Look in "Releases" tab