Hi Bouzahme
There may be other ways, but browse to your SQL Log directory (default C:\Program Files\Microsoft SQL Server\MSSQL\LOG) and open SQLAGENT.OUT. You should see a line like this '2007-09-18 08:33:56 - ? [129] SQLSERVERAGENT starting under Windows NT service control' which tells you when the Agent last started.
Cath