Zkoušel jste includovat metodu přes stdio.h? Popřípadě msdn uvádí _wspawnl pro wchar a ta je určitě v stdio

Použití:
intptr_t _spawnl(
int mode,
const char *cmdname,
const char *arg0,
const char *arg1,
... const char *argn,
NULL
);
¨Return Value
The return value from a synchronous _spawnl or _wspawnl (_P_WAIT specified for mode) is the exit status of the new process. The return value from an asynchronous _spawnl or _wspawnl (_P_NOWAIT or _P_NOWAITO specified for mode) is the process handle. The exit status is 0 if the process terminated normally. You can set the exit status to a nonzero value if the spawned process specifically calls the exit routine with a nonzero argument. If the new process did not explicitly set a positive exit status, a positive exit status indicates an abnormal exit with an abort or an interrupt. A return value of –1 indicates an error (the new process is not started). In this case, errno is set to one of the following values:
E2BIG
Argument list exceeds 1024 bytes
EINVAL
mode argument is invalid
ENOENT
File or path is not found
ENOEXEC
Specified file is not executable or has invalid executable-file format
ENOMEM
Not enough memory is available to execute new process
See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these, and other, return codes.
Remarks
Each of these functions creates and executes a new process, passing each command-line argument as a separate parameter.
Popřípadě můžete-li includovat Qt4, tak tam je na to metoda v třídě QProcess
