The installation process for the 64bit client for both 5.9.4 and 5.9.5 works fine, however, as soon as the BOINC service starts, I get a windows pop-up saying "BOINC has encountered a problem and needed to be closed" and asks if you want to report the error. I do send the error just because it is a beta OS, and perhaps some compatibility issues can still be resolved, if this is indeed the issue. The instillation process never completes for BOINC never successfully starts.
My system is a core2duo e6700, 2 GB ram, nvidia 6800, 975 chipset. I had 5.9.4 fail under server longhorn enterprise x64 and 5.9.5 fail under the standard version. I have no AV, and just windows firewall. No added rolls are installed. If you need any added info, just contact me.