Windows has its problems, but you can't blame it all on the OS. A fresh install of Windows XP on a 10 year old machine won't perform as well as it did 10 years ago because it's expected to do a lot more than it did 10 years ago.

There's kind of an obligation to keep an antivirus running on a network-connected Windows machine, especially a server. AVG used to run reasonably in 64 megs. Now it won't even try to run in 512. Their full-download exe's have more than quadrupled in size in 5 years. And that's one of the less demanding AV's. I can't even imagine what Mcaffee and Norton require these days.

On top of that, much new software will force you to install otherwise-nonessential updates just to function. The smart ones bundle many with them, so they don't even need to ask ( I think this may account for some of AVG's increasing girth ).

So I don't think it's the Windows operating system that's ultimately to blame as much as the Windows software model. There's still people using Visual Studio 6.0, buggy and old as it is, because anything newer adds an entire .Net requirement to any program containing "int main()". If you build your software with MS tools, it will demand not just newer systems, but more runtime software in general.

There's no such continuous push to extend your operating environment in UNIX. Security fixes are one thing, but your distributor's not likely to suddenly decide they really, really like a newfangled language and insist on using it for everything, dragging along its hundreds of megs of footprint, memory-hungry daemon+runtime optimizer+JIT compiler, and endless updates just to do what used to work without it. Usually. (Looking at you, Java. ;p)

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Furthermore, it really does matter what you intend to do with the server. I ran a web and database server with a good number of decently intensive sites on it for several years using an old Pentium III 866Mhz with 512M RAM, with the load average mostly below .1. One instance of Wordpress took more CPU and resources than everything else combined. It's keeping the improved 3Ghz machine at a load of .4 even now.