Here's one way:

NT, 2000, and XP all set the OS environment variable OS=Windows_NT

  If OS is set, then your operating system is NT, 2000, or XP

  If OS is not set then your operating system is 95/98/ME

  If OS=Windows_NT, and the variable "ProgramFiles" is set then you have
  an operating system that is 2000 or XP, else your operating system is   
  NT.

Here's another way:

Define a DOMAIN called DWORD, numeric, 10.0 digits, signed, storage-type = binary. (This is important, it must be exactly that format).

Define a one-record working storage file (I called mine 1EX VERSION) with the following structure (note - each DOMAIN field is defined by the DWORD domain):

     Seq No  Field Name          Field Type Format
     ======= =================== ========== ======================
     100     VERSION SIZE        DOMAIN     S9(10)
     200     VERSION MAJOR       DOMAIN     S9(10)
     300     VERSION MINOR       DOMAIN     S9(10)
     400     VERSION BUILD       DOMAIN     S9(10)
     500     VERSION PLATFORM    DOMAIN     S9(10)
     600     VERSION CSD         ALPHA      X(128)

The following ILF should populate the VERSION record for you:

      SET      --- TEMP 80                    =      kernel32.dll,
      APPEND   --- TEMP 80                    0      GetVersionExA
      *
      SET      1EX VERSION SIZE               =      148
      PASS     1EX VERSION                    RECORD           SHARE? Y
      CALL     --- TEMP 80                    RESIDENT? N  END? N  FAIL 0

If --- RETURN CODE is not equal to 0, the VERSION record should be filled in.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/sysinfo/sysinfo_49iw.asp (all one URL) has more information about the GetVersionEx function and the contents of the OSVERSIONINFO structure.