I would assume that a Dell would come with the HD already partitioned and Windows installed. If that's the case, you will need a special program (such as Partition Magic) in order to re-partition it without erasing everything.
Opinions on how to partition the HD vary, but I prefer dividing mine up. Here is how I have my HD set up:
C: 4 GB Operating System only. This way if something goes wrong with the OS, I only have to restore it, and not my Apps or Data.
D: 2 GB Fixed size Page File only. Prevents drive fragmentation and speeds up access for the virtual memory.
E: 24 GB All Apps. Normal apps in the E:\Program Files folder, Games in a Folder of their own.
F: 35 GB All Data. Easier to back up and restore on a partition by itself.
G: 4 GB Temp. Windows creates hundreds of temp files during various operations (i.e. installing programs, patches), by assigning the Windows temp directory to a seperate partition, it cuts down on fragmentation on the OS partition. My browsers cache is also on this partition.
Edit: Also, I assign my CD/DVD drives the drive letters X,Y, and Z so that they aren't dependent on the number of HD partitions that I have.