Error "Steam is temporarily unavailable, please try later..." while the steam client remains at 0 % completed always and after that shows this message, I have just found the solution which fixes this problem. I formatted my pc and installed Windows 7 x64 Ultimate, after that steam was giving this error, i just tried everything, firewall, ports, deleted clientregistry.blob, appcachesettings.blob, formatted 2 times, registry cleaners etc ... Still THE PROBLEM WASNT FIXED.
I just remembered that before formatting I copied the whole steam folder from c:\program files\steam (earlier I had 32 bit), to some other location (E:\Steam), just in case of need of any setting or cfg or demo/ maps files.
Now I just replaced the old steam.exe that comes with that 1.5 MB setup with this new one. And deleted clientregistry.blob and ran the file without any compatibility mode or whatsoever. It just ran fine, it updated successfully.
So here what you have to do ...
Download and install the steam client from here http://cdn.steampowered.com/download/SteamInstall.msi
After Installing it would have ran automatically and you got the error "Steam is temporarily unavailable, please try later...". Now, goto your steam directory (c:\program files(x86)\steam)
Delete Clientregistry.blob file
Download the new steam.exe file (the newer version that comes after update) and replace it with the already existing steam.exe file in steam folder.
Now I have uploaded the file here http://www.mediafire.com/?xgy2z5rc47x5nrt. No need to worry, the file has been scanned at Virus Total so you can view the summary report. Its completely free of viruses or anything. Just an updated version of steam.exe.
Now just run steam.exe without any compatibilty mode or anything. It will start updating soon (after 1 minute, it wont show any error or anything, it will star updating steam platform files, and you will have your steam client ready to play your favourite games.
DONE! Whew! Finally that error is gone and it installed.
You can view the video also, how to fix this error, or just follow the above mentioned steps, quick and easy.