Based on the context of the original message, I'm guessing Patricia doesn't know what a partition is, or that this can even be done to hard drives (and she wouldn't be alone; I'd guess 99% of computer users who've never done any of their own computer work and have simply bought them and had someone else do all the drive and OS installations, etc., don't know about such things).
So we need to begin by explaining partitions, but I fear that by the time we get where we're going, if Patricia is like the average computer user, she may be either too scared or confused to go any further, and the advice to call the friend who installed the drive and have them take a look may be the best bet.
If not, Patricia, in essence, each "partition" of a hard drive is, as far as the computer's operating system is concerned, a SEPARATE HARD DRIVE. So you could have ONE big hard drive in your computer (and even now 2 TB is still a fairly substantial drive size), but when you look on Windows Explorer it may show 2, 3, 4 or more drives are there, depending on how the drive was partitioned. Think of the hard drive like a big file cabinet. The original drive had only one drawer in it. Each partition beyond one is an additional "drawer."
You may wonder why someone would do things this way. There's good reasoning behind it. For instance, these days I generally just buy a "boot drive" big enough to hold my operating system and all programs I'll be installing, and get the fastest drive I can afford for that (a solid state drive, which operates at speeds close to the computer RAM, or memory -- MUCH faster than standard hard drives). Then I use second, third, etc. hard drives to store all my FILES (pictures, movies, letters, whatever) on.
The reason to do this is that we tend to copy, move around and delete pictures, movies and whatever other "data" files we have, but once we install an operating system or program, it's usually on the hard drive to stay -- at least until we upgrade it. All the "data" files (pictures, and so forth) that get continually copied, deleted and moved around make a sort of MESS of the space on your hard drive and slow down its operation (imagine paper that had pencil marks continually erased and then re-written in different locations -- except sometimes the drive actually splits files into pieces and puts a piece here and a piece there based on how space is available). That's not too bad if that drive has only DATA on it, since programs will still just open and close those files, and every time they re-save them, they will put them "all in one piece again" if they happen to be scattered around the drive in little pieces and they can find a continuous block of space long enough for the particular file(s).
IF, however, you have all your programs on the same drive with all your data files, then it's possible that all the little pieces can end up intermingled with bits and pieces of your programs and even the operating system, like Vista, in your case. When such a thing happens, the read head on a hard drive has to hunt here and there to load what should be consecutive pieces of an operating system or program. As you may imagine, in such a case, this will cause the system to begin running slowly and may even cause it to quit working correctly at all, if things get bad enough (or may cause some programs to quit working correctly and need to be re-installed).
So it's possible your friend created a partition for just Vista and the programs you install, and a second partition for all your DATA files. But the programmers at WINDOWS don't think like I (and many others) do. They still seem to think all Data should go on the boot drive, regardless. So they have Windows set to DEFAULT to set up Windows to put all your "my documents" folders on the BOOT drive (or partition). If your friend set up a smaller partition for Vista and your programs, but didn't bother to ensure all your saving is re-directed to the larger partition, then it's QUITE LIKELY that smaller partition has filled up. Even if you do set up a redirect, Windows will keep trying to force you to put data on the boot drive, so you may have to manually tell it to store on the other drive or partition (which is what I usually have to do). It can be a pain in the butt, but it keeps your system "cleaner" and better organized.
As I believe someone else also mentioned, it is also possible that the system wasn't set up for a drive that large. I don't think I got to a drive that large until I was using System 7, so I don't really know what sort of "dances" might be necessary to ensure Vista will properly accomodate a 2 TB drive, although it may do it natively. I'm pretty sure if it will it's the first version of Windows that does.