The default location and filename of catalogs on Windows 7 systems is:
"C:\Users\[youruser]\Pictures\Lightroom\Lightroom 4 Catalog.lrcat"
Along with it there should be a folder containing the previews called "Lightroom 4 Catalog Previews.lrdata".
If you can't find the catalog there, you might not have been using the default catalog name and location.
- Find the file "C:\Users\[youruser]\AppData\Roaming\Adobe\Lightroom\Preferences\Lightroom 4 Preferences.agprefs"
- Open it using notpad
- Find an entry indicating "libraryToLoad20 ="
- This entry points to the path and name of the catalog that was set to load when LR starts.