List the queries running on SQL Server

You can run the sp_who command to get a list of all the current users, sessions and processes. You can then run the KILL command on any spid that is blocking others.

This will show you the longest running SPIDs on a SQL 2000 or SQL 2005 server:

select
    P.spid
,   right(convert(varchar, 
            dateadd(ms, datediff(ms, P.last_batch, getdate()), '1900-01-01'), 
            121), 12) as 'batch_duration'
,   P.program_name
,   P.hostname
,   P.loginame
from master.dbo.sysprocesses P
where P.spid > 50
and      P.status not in ('background', 'sleeping')
and      P.cmd not in ('AWAITING COMMAND'
                    ,'MIRROR HANDLER'
                    ,'LAZY WRITER'
                    ,'CHECKPOINT SLEEP'
                    ,'RA MANAGER')
order by batch_duration desc

If you need to see the SQL running for a given spid from the results, use something like this:

declare
    @spid int
,   @stmt_start int
,   @stmt_end int
,   @sql_handle binary(20)

set @spid = XXX -- Fill this in

select  top 1
    @sql_handle = sql_handle
,   @stmt_start = case stmt_start when 0 then 0 else stmt_start / 2 end
,   @stmt_end = case stmt_end when -1 then -1 else stmt_end / 2 end
from    sys.sysprocesses
where   spid = @spid
order by ecid

SELECT
    SUBSTRING(  text,
            COALESCE(NULLIF(@stmt_start, 0), 1),
            CASE @stmt_end
                WHEN -1
                    THEN DATALENGTH(text)
                ELSE
                    (@stmt_end - @stmt_start)
                END
        )
FROM ::fn_get_sql(@sql_handle)

If you're running SQL Server 2005 or 2008, you could use the DMV's to find this...

SELECT  *
FROM    sys.dm_exec_requests  
        CROSS APPLY sys.dm_exec_sql_text(sql_handle)  

• More about sys.dm_exec_requests
• More about sys.dm_exec_sql_text

I would suggest querying the sys views. something similar to

SELECT * 
FROM 
   sys.dm_exec_sessions s
   LEFT  JOIN sys.dm_exec_connections c
        ON  s.session_id = c.session_id
   LEFT JOIN sys.dm_db_task_space_usage tsu
        ON  tsu.session_id = s.session_id
   LEFT JOIN sys.dm_os_tasks t
        ON  t.session_id = tsu.session_id
        AND t.request_id = tsu.request_id
   LEFT JOIN sys.dm_exec_requests r
        ON  r.session_id = tsu.session_id
        AND r.request_id = tsu.request_id
   OUTER APPLY sys.dm_exec_sql_text(r.sql_handle) TSQL

This way you can get a TotalPagesAllocated which can help you figure out the spid that is taking all the server resources. There has lots of times when I can't even bring up activity monitor and use these sys views to see what's going on.

I would recommend you reading the following article. I got this reference from here.