Simple question, I hope. How do I or do I need to update BOINC/SETI? Does the system send an update every once in a while when a new version is available? I looked at the download page and it looked like it was for UNIX which I have used but my system is Windows now.

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Don't give up -- The Ark was built by amateurs and the Titanic was built by professionals.

It is you that will need to update BOINC, it won't update automatically when a new version is released to recommended. However, you will see messages about a newer version being available in the event log.
http://boinc.berkeley.edu/download.php will always have the latest available recommended version for your operating system. The page will detect what OS you run.

Mind, it's not adamant to update BOINC. If an older version runs fine, almost without errors, on your system, you can just let it run there.

As for updating Seti applications, these do automatically update whenever newer versions come out. Unless you run third party applications through the anonymous platform (app_info.xml), then you will have to update these applications if necessary.

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