All right, then here is the 1st (?) not one partition post
If you are not familiar with tools such as nLite (XP and older) or vLite (Vista and newer), then I wouldn't make several partitions. But if you do, I always make a partition for the Windows OS itself, one partition for the software I need to install and my portable applications, another one for my data and preferably one partition for temporary files generated by the system and all users alike (including page/hybernate files).
Admittedly, this is a very involved way of installing Windows, but for me the advantages outweigh the pain of this installation method.
Temporary files and user generated files always make a mess of a one partition, requiring a lot of defragging over the life-time from the Windows installation. With one partition dedicated to just the Windows OS files, not much defragging is necessary at all and if you do it is done very fast. Keeping your installed software separated from the rest is also helpful, not much extra data changes are expected, so after a while hardly any defragging is necessary anymore and program files are positioned optimally, giving you between 3 to 10% global performance gain of the hardware.
Separate partitions for C:\ (Windows OS) and D:\ (Programs), you can install Windows fresh and with freeware such as ERUNT and have all your software re-installed within 30 minutes. That is how long it took me to revive a Oracle database server (with 2x 300+ GByte databases ready to run again).
Granted, it's not for the faint of heart, but I have never believed in the MS mantra of dumping everything on one pile and letting 3rd party software clean up the mess. The file system from the Amiga home computer worked much nicer back in the day and is/was much more akin to the Linux way of handling storage media. but you can apply these principles quite well on Windows too, although with each iteration of Windows it is more and more 'only their way or the highway'.
You might have guessed by now, I am highly/deeply/extremely opinionated about who is allowed access to the data I generate on my computer (using a quite rigid folder structure I made that suits my workflow best).
Mouser is right, software like MiniTools makes you shrink the C:\ partition first before you can make the second one. And MiniTools is very decent software. I have used the freeware version of their software on several occasions and always delivered, so you won't hear me say anything negative about it.