I believe that the problem with switching versions of Limewire comes from uninstalling the old version. Limewire taps into the Gnutella network when you search and for some reason alot of the P2P programs that connect to that network screw up your ability to run downloaded executable files after you uninstall them. Executables are necessary for setting up any program and it sounds to me like that is what is failing when ya'll try to open Limewire.
I had the same problem with downloading the new version, so I went looking for an older one and found, like, a million of them at www.oldversion.com. The version I am running now works perfectly except for a few error messages every now and then. I suggest you all go there and see if one of those work for you.