Did install BOINC on my laptop and it is working fine, but it is not working on my new Windows 10 PC. The BOINC manager tells me"Unable to connect to the core client" and the projects box is completely empty. I did turn off my firewall, but didn't solve the problem. I tried everything.

Check in Windows Task Manager if the boinc.exe process is running. That's the client.
If it is running and BOINC Manager still cannot make contact with it:
1. when in BOINC Manager, go to View->Advanced view (if not there already), Options, Other Options, and check that "Run the client?" is checked. If it isn't, check that, do Save, File->Exit BOINC Manager and restart it.
2. add incoming and outgoing rules into the Windows firewall or whichever other firewall you have, for boinc.exe and boincmgr.exe
3. exclude boinc.exe. boincmgr.exe, boinctray.exe etc. from being actively scanned by anti-virus and other anti-malware programs.
4. check that your user account has rights to run both.
5. check that both BOINC Manager (boincmgr.exe) and the client (boinc.exe) start from the same program files directory, not one from Program Files (x86) and the other from Program Files.

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"Check in Windows Task Manager if the boinc.exe process is running. That's the client. "

Thanks for replying. Boinc.exe on my Windows Task Manager under Apps isn't running. I did click on Boinc.exe, but it runs for a few seconds. I don't know what is blocking it, I did allow it in my anti-virus and windows firewall. I do not think my computer is infected with a virus because I just bought it yesterday. I did check 1 to 4 steps, but the project box "Choose a project" is still empty.

Could it be because BOINC was already installed on my laptop? Does BOINC work on two devices connected to the same router? Is BOINC compatible with Windows 10?

I am not sure what you mean in number 5.

Could it be because BOINC was already installed on my laptop?

No BOINC works separately on each computer

Does BOINC work on two devices connected to the same router?

Yes, I have had up to 8 devices all processing SETI@Home work units at the same time.

Is BOINC compatible with Windows 10?

Definately

Navigate to your BOINC Data directory (default a hidden directory at C:\Programdata\BOINC\ so just fill that in into Windows Explorer and hit Enter) and look for the stderrdae.txt file. Set Windows Explorer to show the window with all details. Is this file larger than 0 KB?
If it is larger than 0KB, open it in Notepad and scroll to the bottom.

Look for the last (most recent) occurrence of

BOINC Windows Runtime Debugger Version xxx

Dump Timestamp    : xx/xx/xx yy:yy:yy

- that should be today.

Below that, look for "*** Dump of thread ID...". Copy from there to the end, and post it here.

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I am not sure what you mean in number 5.

This is a possibility where you start the wrong program from the wrong directory. If for instance you installed a 32bit BOINC, it will have installed into the C:\Program Files (x86)\BOINC\ directory on a 64bit Windows. If you then installed a 64bit BOINC, it has installed its parts in C:\Program Files\BOINC\
It is possible to run a client from one directory and a manager from the other, but these won't be able to communicate with each other, as they don't start from the same directory.

But that's a highly unlikely scenario on a fresh computer, so you can ignore that scenario.

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It is 0 KB, and there is nothing in stderrdae. It is all blank.

I am going to reset the PC and start all over.

Still have the same problem. More suggestions will be appreciated.

Okay the 32 bit version of BOINC is working, but I have a 64-bit Operating System. It makes no sense. Is there something wrong with the computer I just bought? Thanks in advance.

64 bit version is now working.