I for one, have always been a proponent of multiple partitions (and even drives) in Windows for many reasons, primarily because I don't want to risk putting all my eggs in one basket.
I've found over the years that creating one partition for Windows and system related apps (antivirus, etc.) to reside in C: drive is the best insurance of a system recovery just in case a crash of C: drive happens. Further, to enhance performance (and reliability), I also create two small partitions (~10GB), one dedicated for the swap file (D: drive), and another dedicated to TEMP/TMP files (E: drive), internet browser caches, etc. I also use this for "temporary" file use, fiddling, and such, where the key use is "temporary".
Also in my disk management scheme, I then create several partitions for dedicated use, such as games, apps (Office, etc.), all personal docs and data, pictures, and music. This is really useful in keeping all these types of files manageable, and also makes backup of critical data easier to maintain. I round things out by assigning my DVD/CD drives as last drive letters, such as Y: and Z: drives. I also reassign temporary pop-up drives (such as printers with internal card readers) to the tail end as well (i.e. X: drive).
The dedicated swap file partition (D: drive) is (at least to me) important in that Windows creates, deletes, and (sometimes) resizes this file, and I really don't want it to interweave itself into C:. If you have a lot of RAM, this file may not even be accessed, but it just depends. Having the swap file as the only file in that partition, makes access to the file that much faster.
I also usually install a second physical drive used for very large files, and also as a pseudo RAID drive, just so I have a physically separate drive to store critical data. This is also a good spot to put your Windows system backup on. Additionally, if you have apps that do use temporary files at a heavy rate, placing the swap and temp partitions on the front end of the second drive will also aid in performance, where the system is R/W data separately from the application files on the primary drive, and not having the hard drive seeking data all over the drive (especially compared to one single monster partition.
Now, I'm not saying that works for everyone, but it works for me, and (ever since Windows for Workgroups 3.1) its saved my bacon more than once.
Finally, as for resizing and re-partitioning, my new Gateway PC came with a huge 1TB drive. In Windows 7, they do offer a method of resizing a partition, but it seems to have some serious limitations. On initial setup (after doing a first pass system backup), I tried to resize C: from the initial 1TB partition to 100GB (more than adequate of Windows and system related apps). Unfortunately, disk manager would only shrink it down to 500GB, way to big for its intended use. After much trial and error, the only option left was a third party partition manager app.
Knowing that this is really a one-time-only requirement, I looked at many apps, and found
EASEUS Partition Master 5.0.1 Professional Edition as a good and reliable app for resizing and repartitioning for Windows 7. In fact, if you're running Win7-32 bit, they have a "home edition" available for free! For 64 bit, the pro version (the one I have) is $40. No, I have nothing to do with EASUS, just giving my review of it. It was worth the $40 to save all the frustration with Win7's built in disk manager.
I hope all this helps someone. It may seem like a lot of extra work, but it surely works for me.
Bill