Put your installation disc in the drive.
Press F1
Go into Boot Up menu.
Then there's a "Boot Order" section (I'm not going in to find out exactly what it's called)
Select CD Rom as your first device.
F 10
Then "Press any key" when prompted.
No need to re install.
Just do a repair start up or repair installation.
If you've just added another hard drive, by the way, disconnect or reformat it. It may be buggering you up.