You can only have 4 primary partitions on a hard drive. You can make one extended partition to take the place of one primary partition. Inside of the the extended partition, you can create 20+ logical partitions.
If you already have four primaries, you will have to delete one (probably the last one you made) and then create an extended partition.
As for skibbi, I currently run 6 operating systems on one drive, which forces me to have several partitions. However, if you are only a Windows user, having many partitions may not always make sense.
There's probably an AnandTach FAQ covering this subject and I couls always repost my Dissertation on Parititoning.
-SUO