I always divide up any hard disk in at least 3 partitions, but my preference is 4 of those.
One partition for Windows (and literally nothing else).
One partition for software I install.
One partition for data.
One partition for temporary storage.
On the Windows partition I move every user-related folder to my data partition using the Windows options that are available to me.
On the Windows partition I move every temporary storage-folder (user and system) to the temporary storage partition using the Windows options that are available to me.
On the Windows partition I make the page file 8GByte in size and won't allow it to grow above that. On a normal PC I disable hibernation.
Whenever I install software, I select the software partition as it's home. And because I am the only user, I use portable versions of software I wherever I can. And if that is not an option, I have some software that allows me to make portable versions of software that requires installation normally.
Why do I go through all that trouble?
The Windows partition is hardly ever written to (besides updates). This reduces the need to defrag it drastically. And the Windows file system will after some time have placed the files in their optimal position on that partition. This way you can make the Windows partition lean and mean. And it will stay that way. I normally allocate about 25GByte for the Windows partition.
The same is true for the partition that contains my software, especially when you don't install new software on a regular basis. I usually allocate 100GByte for the software partition, which is enough, because I make sure to configure applications to work with and store everything on the data partition. I must add that I hardly play any games anymore.
As a result the data partition and temporary storage partitions get a bit messy. I usually allocate 25GByte to the temporary storage partition, mainly because I don't think I need that much garbage on my system to begin with. The rest is for the data partition. I don't care too much about the content of the temp partition and wipe it clean at least once month. Saves me defrag session as well
That leaves the data partition, that one does need a defrag once in a while, but not nearly as often as you think. And even on that partition the Windows file system will find the optimum location for the content after some time.
Exaggerated? Perhaps, but in my mind all of the above makes perfect sense. There are many schools of thought regarding this subject and most are there on merit. Just couldn't help adding my