I spent the better part of an afternoon & evening trying to get the latest Arduino IDE to work with Windows 10 (64bit) & this is what worked (and didn't) for me.
I installed IDE 1.6.8 which did not work - giving me a er_open(): "can't open device "\.\COM6 alert.
Scoured the forum, internet - tried numerous things including driver updates, uninstall, restart, re-install - nothing worked.
(Win 10 kept telling me that the proper driver was the one that was currently installed - I did try to change the setting to allow unsigned drivers to be installed - but did not have any luck getting that to work either)
I had taken a hacker-space Arduino class a few years back & remembered the presenter mentioning he never used the latest IDE - always one or two revisions back.
I was about to give up, but decided to uninstalled 1.6.8 & reinstalled 1.0.6 -and voila - worked first time around - had it up and running in less than 15 minutes - still works this morning.
Don't know if this will solve everyone's similar problems - but it definitely solved mine & felt I had to share.
I did do a Linux IDE install on my Ubuntu machine & checked both of my Arduino Uno's to rule out any issues there - they both worked fine.
1.0.6 is really old, but 1.6.5r5 has the key features of 1.6.x versions, without the bugs they introduced in 1.6.6 and later (IMO, 1.6.6 and 1.6.7 should not have been released in their current state, and it looks like 1.6.8 is another stinker)
That said, that's a VERY weird problem to be fixed by changing versions. Unless there's a new bug regarding treatment of serial ports in 1.6.8 - I've seen a few reports of inexplicable access denied errors (is that the error you're getting? Cannot open COM6, access denied?) since it came out.
I realized it was an old revision, but after 5 hours of not knowing whether it was Win 10, the Uno's, or the IDE - I was tired & just giving it a last chance (which worked)
Was trying to get my Uno to work with MAX MSP using Maxuino & Firmata which now work perfectly (even with the ancient revision)
yes - I can't remember the exact error wording, but basically it could not communicate with the arduino on COM 6 / I did try using another unused port - but that didn't work either.
I had a text dump of the error, but did't save it once everything started working
I had this exact same problem, so I wanted to bump to report that this thread solved my problem after exhausting other threads where i'm messing with drivers and switching com ports who always said they were being used by another program.
I have Windows 10 64bit home ed. I had to reinstall due to some other issues and wanted to start fresh on my primary drive. The symptoms were after installing the IDE again, no matter what com port I tried I got access denied.
I changed com ports, same problem, I did eventually uninstall and reinstall the drivers through device manager and was able to upload once, then after that, couldn't upload anymore and got Access denied on whatever com port I was using. Had the 1.6.8 IDE, went back to the 1.6.5 and it completely solved my issue.
Thank you for Rotwang11 for posting and DrAzzy for the solution. It was incredibly frustrating to not work on my project.