add serial number begining of each line in a text file

Dear All,

i am having text file below

rama 123 done
raju 456 not done
keshav 123 done
...............
............

i want to add a serial number to this file, the output should be

1 rama 123 done
2 raju 456 not done
3 keshav 123 done

99 ...............
100 ............

can anybody help me

Try this

sed "=" test1.txt | sed "N;s/\n/ /"

one more apart from CarloM's attachment..

$ sed = infile | paste -d' ' - -