When you install a program and you have 2 partitions, say a C:\ and a D:\ drive, and your C:\ drive is your OS drive, you don't want to install ALL your programs on it since, as most have said, they make it just 8 GB or so. This would quickly lead you to running out of space. Instead, you install the program to the larger partition. Some part of the program will go into the C:\ drive (windows system files, etc) but the bulk of the program will go into the D:\ drive. But you have to tell it to do that, otherwise it will just install to the C:\ drive. Most people don't do this, they are clicky types who just install to the default directory. My point being that if you are going to partition the drive, just make sure that when you install programs (not all mind you, say like games, etc), make sure they get installed onto the bigger, non-os partition. The other thing is that you may want to also change the pagefile to the non-os partition, as it grows and shrinks to accomodate memory usage unless you specifically tell it to stay one size. Nothing sucks more than getting an error because you are out of space on your OS partition, and then find out you have gigs of space on your other partition.
So, to reiterate (so I can understand it also 🙂 ), partitioning your hard drive isn't a bad thing, you just have to do a couple of thing differently when installing programs. I actually find that it's a pain in my @ss (tech support wise) if I do this for someone, as they usually don't follow the rule of not installing everything on the OS partition, and I then have to resize it for them or transfer programs to the other partition. All in all, much easier to just make it one partition for them. I don't exactly deal with the most computer literate of people, and I find that this works out better for both them and myself.
So, to reiterate (so I can understand it also 🙂 ), partitioning your hard drive isn't a bad thing, you just have to do a couple of thing differently when installing programs. I actually find that it's a pain in my @ss (tech support wise) if I do this for someone, as they usually don't follow the rule of not installing everything on the OS partition, and I then have to resize it for them or transfer programs to the other partition. All in all, much easier to just make it one partition for them. I don't exactly deal with the most computer literate of people, and I find that this works out better for both them and myself.