<p>This is just a general question - I was sitting and waiting for a bit of software to compile (we use Incredibuild here but can still take 10/15 mins) and it got me wondering, does anyone know how long it took to compile Windows XP or Vista? </p>

<p>I did some googling but didn't really find any useful information</p>

Third-hand information I have is that it takes about a day to complete a Windows build. Which is more or less in line with attempting to build your favorite OSS Operating System from scratch.
Building a modern operating system is a complex and difficult task. The only reason why it doesn't take longer is because companies like Microsoft have build environments setup to help automate integration testings. Thus they can build a system with less manual effort than is involved in most OSS builds.
If you've like to get a feel for what it takes to build an operating system, might I recommend the free eBook:
Linux from Scratch
For a more automated build, try
Gentoo
. Both options should give you a better idea of the Operating System build process.