Open source software and free software are two different things, sometimes you get an open source software but not for free, some other time you get a free software but not open source so we should not mistake these two approaches. Red Hat is an open source software but it is not free, and its aim is not to be used on desktop pcs however you can use it on a desktop, so here we have an opensource but not free software, but as it is open source so you can get the source code. there are many other free software which are not open source an example for it can be the Yahoo Messenger.
Now for operating system just consider the free web host you are using it right now, if you see in cpanel server is based on linux operating system, i can say many servers in the world are based on open source operating system so saying no to an open source operating sytem because microsoft is existing is not a nice thing to do. i don't talk about mac because who is using that ?

really if i have to go for not opensource i will go for windows, who installs mac on a PC or notebook that apple doesn't produced it ? and how do you want install when most of the times they only sell the update.
So here is what i think : if i'm going to my university they i need some software to use, one of them is Matlab and another is AutoCad , i have to use them and i have to give my projects in their specific formats. Autodesk is not producing AutoCad for linux but Matlab has a linux version, if i want to use linux i can do two things: 1.i have to buy those two (Matlab & AutoCad) and i run them in linux through Wine with so much decreased performance and many other problems 2.i can use alternative softwares like Octave as an alternative for Matlab and QCad as an alternative for AutoCad, i should say that none of these alternative have the abilities of the main software (not half of it even).
Which one of those two ways i've mentioned i choose, i will face real problems, so i prefer to spend a little much and get a windows seven (or any other version) to enjoy my life without suffering of inadequate abilities or performance of using alternatives or windows softwares in linux. i have shown you sometimes even with you being interested in open source you can not insist on using it because when it comes to open source many people start to thinking about it being free, this type of thinking will make developers a little not interested in producing a software because the developers are human too and they need to make money for their daily life so they have to do open source things (those that are free) beside their daily work and life and this is the difference between commercial software developers and
free open source software developers.
In the end i admit using free operating systems like ubuntu or many other distributions is really enjoyable but sometimes you really cant use then and you have to accept that, but if everything goes into cloud computing then i think windows will not have any advantage over linux and then you will see how linux users grow. now i'm having both windows and linux in my PC and as linux is more secure for surfing web i always use it but for doing my university projects i have to use windows.