How about something like this, you can choose how you display..
select NodeID,IP_Address,Caption,Status, StatusLED, LastSystemUpTimePollUtc,
DATEDIFF(mi,LastSystemUpTimePollUtc,GETDATE())% 60 as MinsDown,
DATEDIFF(hh,LastSystemUpTimePollUtc,GETDATE())% 24 as HoursDown,
DATEDIFF(d,LastSystemUpTimePollUtc,GETDATE())as DaysDown,
--OR
convert(varchar(10),DATEDIFF(d,LastSystemUpTimePollUtc,GETDATE()))+':'+
convert(varchar(10),DATEDIFF(hh,LastSystemUpTimePollUtc,GETDATE())% 24 )+':'+
convert(varchar(10),DATEDIFF(mi,LastSystemUpTimePollUtc,GETDATE())% 60)as'DD:HH:MM:SS',
--OR
convert(varchar(10),(DATEDIFF(d,LastSystemUpTimePollUtc,GETDATE())))+' Days '+
convert(varchar(10),(DATEDIFF(hh,LastSystemUpTimePollUtc,GETDATE())% 24 ))+' Hours '+
convert(varchar(10),(DATEDIFF(mi,LastSystemUpTimePollUtc,GETDATE())% 60))+' Mins 'as'DD:HH:MM:SS'
from nodes
where StatusLED ='down.gif'