Several ways to do this.
1. Enter a 1 in row 1. Right-click on bottom right corner of cell and drag
down to row 100. Release right button and select "Fill Series"
2. Enter a 1 in row 1. Edit>Fill>Series>Linear>Step=1 Stop=100
3. Enter a 1 in row 1 cell, a 2 in next cell down. Left-click and drag down
to row 100.
4. In row 1 enter =Row(). Drag>copy down to row 100.
Gord Dibben Excel MVP