What media did you install SQL originally from? Was it a disk? A single executable? It is hard to tell you where to install it from without knowing what media you have first. For example full SQL Server (Standard/Enterprise/etc.) normally comes on a disk with several files, but I believe SQL Server Express comes wrapped in an executable.
What installation media do you have available? If it is a single file list the exact filename.
Joie Andrew
"Since 1982"