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Check what's running in the background.

Oh crap. I had this happening to me a while ago.
I struggled to find the problem for weeks, until I realized that it was my graphics card overheating. Installed a fan and never ever had any problem since.

Don't hit alt and tab at the same time while playing?

(have to start with the easy stuff, lol)

Link with people who had the same issue

check your temperatures.

Without further information, (such as processes running at the time), my very simple guess would be Antivirus possibly detecting something, or something that at least it needs your input on or it's something you need to see through the pop-up messages that some AVs use, such as an actual infection on your system

a game minimizing should not be your gpu over heating as that would causes graphic artifacts/freezing/crashes

it seems like you might have some kind of alert or warning

like if you fail to register windows or choose not to with your key after awhile about every half hour it will pop up on the task bar reminding you to activate windows and if your in game in most cases it will minimize it

you could also be hitting the windows logo key which will minimize it

check ot see if you have anything on your taks bar pop up usually it will show a little flag on the right corner of it for windows updates or other issues

but at this point it does not seem to be anything of concern other then being quiet annoying

you can also check to see what is running in the background and what may be trying to update

is possible you have "windows" key on your keyboard blocked, so when you play and use "wasd" "windows key + D key" = Desktop.... and minimize all windows!

it happens to me too :(
i can maximize again but it quite disturbe my gaming time :(

I'm pretty sure your pc is alerting you something, updates maybe?

Check video card temp and CPU temp.

Do you maybe have MS Office installed? Question may be strange, but your problem is similiar to this, and the solution for the OP of that topic was to uninstall Office. OneNote was the one responsible in this particular problem.

EDIT: Also, what is the antivirus you are using? AVG perhaps?

EDIT2: You may want to check your startup apps/processes in msconfig autorun section (WinKey + R → 'msconfig') and check all of your programs that run on startup. Should you find anything suspicious google the name of it and if it's something redundant disable it and see if the problem still exists.

Do a virus scan of your entire PC, it may got bugged also.

If it is game on steam, check consistency of the game files,if it does not help, check (if you can, change) compatibility to older windows.

@edit:You can install again that game.

You have unlicensed Win 8, dontcha :P

It'll pop up every 15 minutes reminding you that your free 1 month license expired and you have to register your copy on Microsoft.

Do you use a special keyboard, like one with extra buttons or some extra software? Maybe there are some extra key prompts for your board which put you back to desktop... at least this has happened to me a long time ago.

Skype sometimes causes this problem. I can't see the alerts over the game but I can still click on them. Also, when I'm in a group chat and someone new is added it likes to minimise games.

Yeah, it could be skype, maybe iTunes update notification, anything like that could minimize your game.

Never had such a problem, so i can't help you. Thanks for the giveaway, though...

Are you using an Xbox controller?

Definitely make sure to get rid of any programs that you're unsure of, as they may be attempting to stay on top of the desktop, along with a virus scan, which you've probably already done. Other than that, little I can say. Sorry.

we will need more details

as like what you have running while your playing a game not just steam and the sorts

but what other things are running in the background

with no further info it is just a crap shoot of guessing what it might be

some video of what happens or a picture of what happens when it is triggered

to be honest your lack of any more info it makes me think it is due to you not having a valid key for windows and every so often it will trigger a reminder to enter a valid key and if i am wrong fine

but if that is the case you more and likely will not get much help with that

anyhow i posted earlier all the most common things it could be and several others have giving ideas but you need to post what you have tried and more info on the issue at hand

it could also be Java asking for an update i have had that happen before

I had the same problem, it becomes unbearable when playing something online.
Unfortunately I don't think I came up with a solution, I don't remember antivirus programs helping, I wiped my HD and reinstalled my OS, I needed to anyway and I recommend you do the same.

Windows 8 have problem when gaming (mine too, so i switch back to Windows 7)
Idk if Windows 8.1 still have problem when gaming too, but i think its seems from uncompatible software. Or check your keyboard/mouse/controller configuration, maybe some setting should be reconfigure.. GL :)

you tried killing all unnecessary background processes? Overheating of something wouldn't minimize it,can't think anything else apart from that..