When I try to install any sourcetree version I get an error that the digital signature of the disk1.cab file is invalid. This started last week when I tried to update to 1.5.1. When it didn't work I tried to reinstall with my old installer for 1.4.x. The old installer gave me the same error. I just tried again with 1.5.2.
I am on an enterprise windows 7 image with pretty tight restrictions. It seems to be that the certificate you signed with can't be verified. I was able to install the 1.4.x version about a month ago so it would seem something has changed with the certificate.