Hi and welcome to the board.
I alter your code a bit because every If should have an EndIf. The return was not needed. Also you mapped the J drive twice. I changed it to K and L as stated in the comments above the line that deletes and maps the drive.

You should check the printer names because the one you want to set as default is not added or there might be a typo in the name.

 Code:
;*************************************************************************
;***                        Main logonscript!                          ***
;*************************************************************************

;**********************************************
;Aanmaken van logbestanden
;**********************************************

$DUMMY = SetTitle("Loginscript")
CLS
? "Momentje Roy, uw loginscript wordt uitgevoerd..."

? "Algemene netwerkkoppelingen worden aangemaakt..."
;*********************************************************
;Netwerk koppelingen
;*********************************************************

;Public data K:
Use K: /DEL /PERSISTENT
Use K: "\\*******SERVER*******\Company"

;Public data L:
Use L: /DEL /PERSISTENT
Use L: "\\*******SERVER*******\Company\Documenten"

? "Printers"
;*********************************************************
;Printers
;*********************************************************

;installation of printers
? "Printers installeren"
If @INWIN = 1 
	If ReadValue("HKEY_CURRENT_USER\SOFTWARE\CEXP\Printer", "Installatie") <> "3"
		$DUMMY = AddPrinterConnection("\\******SERVER******\hpclp3600")
		$DUMMY = AddPrinterConnection("\\******SERVER******\HPLaserJ")
		$DUMMY = AddPrinterConnection("\\******SERVER******\hplj4100")
		$DUMMY = AddPrinterConnection("\\******SERVER******\hplj4250tnplc6")
		$DUMMY = AddPrinterConnection("\\******SERVER******\samsung_clp510")
		$DUMMY = AddPrinterConnection("\\******SERVER******\XeroxWCM")
		$DUMMY = SetDefaultPrinter("\\******SERVER******\hplj4250TN_PCL6")
	EndIf
EndIf
_________________________
Mart

- Chuck Norris once sold ebay to ebay on ebay.