Login to SAP through Windows command prompt

Start -> Run -> CMD,
execute the following command
sapshcut.exe -system=SID -client=CLIENT -user=NAME -pw=PASSWORD

where
SID - 3 characters system ID.  i.e ECC
CLIENT - Logon client.  i.e 800
NAME - Username
Password - Logon Password

Note : SAP GUI must be installed in your system.