When I open My computer I have A: (Floppy disk drive) Local disk C: and local Disk D: The two local disks are due to the single hard drive having 2 partitions. It would be the same if there were 2 separate hard drives each with 1 partition.
In my case. the CD/DVD-ROM is drive E:
If there are 2 operating systems on the drive it may be a dual boot system i.e. you can boot into 2000 or the other (no mention of what it is). This may be why there are two partitions or even two hard drives - 1 O/S on each.
What you do about these depends on wether or not you want to have 2 o/s on the same PC. Did you put them bot on or were they on when you got the PC?