Is there any SW which is capable to access the remote system without knowing the user? I knew Teamviewer, Skype but they require to authentication from end user. I need opensource only for windows..

Are you asking is there any software that you can remotely view the end users computer without the users knowledge? Are you looking to install something on these boxes without the end users knowing? Do you have any privs on these boxes?

Are you asking is there any software that you can remotely view the end users computer without the users knowledge? - Yes
Are you looking to install something on these boxes without the end users knowing? - Yes
Do you have any privs on these boxes? - Yes

You are not going to get those questions answered here. You might want to do a google search on hacking forums.

I'm assuming you require these info for good intention only.

By going on your original post...You dont want to wait for user to give you authentication? If this is the case,yes you do get such legit software however the user will get a notification on the bottom right of his screen stating something like:
"user: sittas87 connected"

No action will be required from user side you will connect instantly

Software: DamewareMini(DamewareMini Pro)

i dont know if this is freeware but you can try the trial- Works similar to VNC

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