I was linked to this thread by someone else, and after reading, I decided I should contribute my opinion to it.
I use Windows XP, and prefer to use it, and will probably continue to do so until none of the newer games I'm actually interested in will work. If the programs somehow stop working earlier than the games, then I'll just use Linux for that.
There is probably a security risk from still using Windows XP, but I am not really concerned. I don't use Internet Explorer (assuming that the latest version is Vista/7-exclusive), and I don't attract viruses. I don't use an anti-virus or a firewall (unless a NAT'd router counts), and I rarely bother to use Windows Update, except after a fresh install. However, I DO use Spybot Search and Destroy occasionally. With all these security issues I give my computer, I have never been attacked during the 4 years I've used Windows XP (and Windows 2000) in this way, except for one time when my computer started acting strangely in general (windows randomly flipped between themselves), so I just reinstalled XP on a completely reformatted hard drive.
I doubt my computer will get attacked tomorrow or next week, and if it does, then I'll just reinstall everything; it is that easy for me, considering I reinstall the operating system once every 6 months anyway. With that said, I am aware that there ARE some risks that cause your computer to be infected with a certain few things you don't notice at all (except for perhaps sluggishness), but I take the risk as I have a few general issues with the various security software I've tried previously, but I won't go into detail on this.
So, why do I refuse to use Windows Vista or 7? They are quite bloated. They suck up a lot of memory and CPU resources for the new features I do not use or care about (eye candy is quite subjective). You might say that newer things will gradually use more and more resources, and that I should be upgrading, and while I do agree that more computing power is good, there is little loss in useful functionality the less resources an operating system uses (for example, XFCE and Fluxbox with Linux, and I quite like them). I would like to be allowed as much resources as possible for running the other, useful things I do use.
The newer Windows do have some new hardware-related features that XP does not have, but I do not use them. I think someone claimed that IPv6 was one of them, although I have no trouble setting it up on XP, and it was one of the reasons I switched from Windows 2000 in the first place. However, and I'm probably contradicting myself at least a bit by saying this, but I will not deny that 64-bit Windows XP may be terrible, although I have yet to try it, and the motherboard only handles 2 GB of RAM (ironically enough it still has a 64-bit CPU) and I don't care to spend more money for a newer computer when this one works quite well, and the games I play run at a high FPS on it.
Much of the latest and newest things still work on XP (and Windows 2000 for that matter!), and so I am quite happy with Windows XP. I should note, as well, that I am not that big of a gamer so I haven't encountered any new games that does not work on XP.
So, Windows XP is old, but not too old to be unusable, and it is not entirely obsolete.
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