Hi and welcome indeed.

BOINC is available outside WCG in 64 bit, but it being irrelevant since the work is done by the science applications i.e. the 32 bit client happily handles any available 64 bit sciences.

In the future WCG will offer more 64 bit science apps, where there's only 1 now [C4CW]:

1. They do have a greater throughput for most, but less for some as well, yes really, so a per device test is needed to discover which of the 2 is best. This test is not yet in place, so when there is a 64 bit science app, the 64 bit will be send to a 64 bit OS, ignoring the BOINC bit compile.
2. There are more systems that are what is termed ''pure'' 64 bit... cannot take 32 bit apps. Those at present only can participate at WCG through C4CW, i.e. the 64 bit compiles of other sciences will allow these ''pure'' devices to have a wider WCG participation.

Summary, you don't need a 64 bit client and funny enough, the Linux repository build is 32 bit for Ubuntu, though the OS is 64 bit. There's no crunching benefit.

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To address the error/warning, cant remember to have seen this reported before on a Windows 64 bit OS at WCG forums. You can right click the installer program in File Explorer and then choose [troubleshoot compatibility, which then offers to execute in XP-SP2 mode on my W7-64 system] or set to execute in a compatibility mode in properties, but from petnek's linked page you are probably better off in using 6.12.34, though there are sparse issues for some using this release on WCG. These having an issue either download 6.12.33 or the latest 6.12.43 [not yet elevated to ''recommended'' level]. Just change the http://boinc.berkeley.edu/dl/boinc_6.12.34_windows_x86_64.exe link to whatever point release you want to fetch, so for 6.12.43 http://boinc.berkeley.edu/dl/boinc_6.12.43_windows_x86_64.exe

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P.S. Please don't take the 7 version... not recommended, in fact still has substantial problems [of course there are always those that see no problem and post that, but that's no guideline... just the lucky]. As of yet there is no formal testing cycle opened that I've seen with intend to convert to declare it a production release... more point releases are being pipelined. WCG for the coming time will stick to 6.10.58 of which the 64 bit link then would be http://boinc.berkeley.edu/dl/boinc_6.10.58_windows_x86_64.exe