First create the folder that you would like docs spooled to then navigate to the key below and edit the value to the new folder you have created.  If you enter the data incorrectly the default path of system32\spool\PRINTERS will be used.

If you would like to change the spool directory for a single printer, just change the value of 'SpoolDirectory' for the printer in corrisponding key below the "Printers" key. 

Key:[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Printers]
Value Name: DefaultSpoolDirectory
Data Type: REG_SZ
Data:Full path to printer spool directory (ie, c:\spool)