LRViewer has been withdrawn because it has become impossible to keep up with changes to Lightroom. However, because it might be helpful in recovering previews, the following links may be used to download it:
LRViewer for Mac OS X
LRViewer for Windows
No support at all is available for these downloads.
Warning: Users report various problems using LRViewer with the latest versions of Lightroom. However, there's no danger of it damaging your Lightroom catalog, so go ahead and try it if you want.
If you're on Mac OS X, File Juicer can also retrieve previews. I don't know of anything like it for Windows, but you may get somewhere with BitmapRip, which I haven't tried. If you're desperate, copy the Lightroom "lrdata" folder to a Mac and run FileJuicer. This may be worth doing even if you have to pay a consultant to do it.