Windows Version

He want's to store it in a variable so I assume he's coding something and therefore needs a commandline tool, not something with a GUI.

As far a NSIS goes, try looking in the help file in Appendix C, fifth script down.

Here it is if you still have issues:

; GetWindowsVersion
 ;
 ; Based on Yazno's function, http://yazno.tripod.com/powerpimpit/
 ; Updated by Joost Verburg
 ;
 ; Returns on top of stack
 ;
 ; Windows Version (95, 98, ME, NT x.x, 2000, XP, 2003, Vista)
 ; or
 ; '' (Unknown Windows Version)
 ;
 ; Usage:
 ;   Call GetWindowsVersion
 ;   Pop $R0
 ;   ; at this point $R0 is "NT 4.0" or whatnot
 
 Function GetWindowsVersion
 
   Push $R0
   Push $R1
 
   ClearErrors
 
   ReadRegStr $R0 HKLM \
   "SOFTWARE\Microsoft\Windows NT\CurrentVersion" CurrentVersion

   IfErrors 0 lbl_winnt
   
   ; we are not NT
   ReadRegStr $R0 HKLM \
   "SOFTWARE\Microsoft\Windows\CurrentVersion" VersionNumber
 
   StrCpy $R1 $R0 1
   StrCmp $R1 '4' 0 lbl_error
 
   StrCpy $R1 $R0 3
 
   StrCmp $R1 '4.0' lbl_win32_95
   StrCmp $R1 '4.9' lbl_win32_ME lbl_win32_98
 
   lbl_win32_95:
     StrCpy $R0 '95'
   Goto lbl_done
 
   lbl_win32_98:
     StrCpy $R0 '98'
   Goto lbl_done
 
   lbl_win32_ME:
     StrCpy $R0 'ME'
   Goto lbl_done
 
   lbl_winnt:
 
   StrCpy $R1 $R0 1
 
   StrCmp $R1 '3' lbl_winnt_x
   StrCmp $R1 '4' lbl_winnt_x
 
   StrCpy $R1 $R0 3
 
   StrCmp $R1 '5.0' lbl_winnt_2000
   StrCmp $R1 '5.1' lbl_winnt_XP
   StrCmp $R1 '5.2' lbl_winnt_2003
   StrCmp $R1 '6.0' lbl_winnt_vista lbl_error
 
   lbl_winnt_x:
     StrCpy $R0 "NT $R0" 6
   Goto lbl_done
 
   lbl_winnt_2000:
     Strcpy $R0 '2000'
   Goto lbl_done
 
   lbl_winnt_XP:
     Strcpy $R0 'XP'
   Goto lbl_done
 
   lbl_winnt_2003:
     Strcpy $R0 '2003'
   Goto lbl_done
 
   lbl_winnt_vista:
     Strcpy $R0 'Vista'
   Goto lbl_done
 
   lbl_error:
     Strcpy $R0 ''
   lbl_done:
 
   Pop $R1
   Exch $R0
 
 FunctionEnd