With vista and windows 7, athough fairly easy, is not straight forward. You can use the "shrink volume" in disk managment. The part that is not straight forward is that it may not allow you to shrink the "c drive" by very much. This is normally do to hibernation file which is a hidden file and normally stuck toward the end of the HDD. Basically you need to disable hibernation, reboot and then run Disk clean-up to remove the dot sys file. Then perform the shrink operation.
See link for a full discussion of "How to". It's for vista, but also applies to Windows 7. I have used this method on a (UGH) store bought - wife's" system and a Laptop. HDD to create a "C" drive and a "D" drive. Don't forget that after you shrink your C drive, you will need to use disk management to set up your "D" drive.
I don't think it's a good idea to separate the drive into 2 volumes. Almost inevetabley you will be looking for more space in oneof them, and then you will post "my drive is full, how to I expand the partition". The only time this would work normally is if you make the OS partition as small as you can safely and install all of the programs and files to the other partition. So your C: drive would be about 3gig over what the OS requires for temp files and swap file.
With a small HDD, under 250 gigs, I would aggree with you. But most "new systems come with 300 + gig HDDs (the 500 Gig becoming more standard). I much prefer a two drive system. I normally set the "C" drive at 100 gigs, which is generally larger than an average user needs for Operating system and programs. Then I use the D drive for all my data/files - it is a big Plus when it comes to back-up. I even partition my small SSD (128 Gig) in my laptop - 60 gigs for Operating system + programs (could have set it smaller as generally only use 30-40 gigs on laptop). Then the other half for my files/data. Before I put any thing on the "D" drive, I set the Windows swap file (min/max set to the same value) to the D drive. This prevents (a) the swap file from shrinking/expanding, and (b) keeps it from Being STUCK in the middle of my "C" drive.