I tried that and I now get a message indicating the LR 4.1 needs to be installed. Oy Vey! So it seems like LR 4.1 got wacked by the LR 4.2 installer and the System Restore wasn't able to restore LR 4.1 properly.
Which, if you think about it - makes perfect sense:
1) you installed LR 4.2 and opened the catalog, converting it
2) you have a system issue, and restore to a pre-4.2 state
3) attempting to open the catalog results in a confusing error message - LR is now a mix of registered and unregistered components, I'm not suprised you would get a 4.1 specific message.
You probably didn't need to uninstall 4.1, but no harm done, the key is re-installing 4.2, allowing you to open the catalog....
Btw - The Lightroom installer does set a system checkpoint.....