If Picasa goes into an update loop
Google no longer supported Picasa after March 1, 2016. If you are using the latest and last version (Picasa 3.9.141 build 259 or Picasa for Mac version 3.9), there is no longer any need to check for updates.
Disable the option to check for Picasa updates
In Picasa3, go to Tools > Options > General tab.
Under Automatic updates, change it to "Don't check for updates"
Click OK
Install the latest (final) version of Picasa
The update loop may happen if you are trying to install Picasa version 3.9.140 build 239. Please don't install that version because some files are missing in the installer. Install the latest version 3.9.141 build 259. See this link
These old directions may or may not be relevant after March 1, 2016, when Google stopped supporting Picasa
Start Windows File Explorer
Paste the following path into the Windows Explorer address bar:
Windows XP: %userprofile%\Local Settings\Application Data\Google\Picasa2\temp\
Windows Vista / Windows 7 / Windows 8 / Windows 10: %LocalAppData%\Google\Picasa2\temp\
In the Picasa2\temp folder, look for a folder called LifescapeUpdater
If the LifescapeUpdater\ folder exists, then delete it.
Paste the following path into the Windows Explorer address bar:
Windows XP: %userprofile%\Local Settings\Application Data\Google\Picasa2\update\
Windows Vista / Windows 7 / Windows 8 / Windows 10: %LocalAppData%\Google\Picasa2\update\
In the Picasa2\update folder, look for a folder called LifescapeUpdater
If the LifescapeUpdater\ folder exists, then delete it.
You may have to update one more time.