Hi, not sure I have understood what you stated. The C drive will normally be the first and active partition of Disc 0, whilst the unallocated space will be the second partition.
Just format the unallocated space via Disc Management so that it becomes useable. I would not be overly concerned about the unexpected partition. As mentioned above, the space may be used, and as it keeps the C drive quite small, defragging will be quicker.
If you wish to get back to one partitioned drive, you will need to reinstall XP and set the partition size during the install. Altewrnatively, use a partitioning program such as PartitionMagic which will keep all your data. Some partitioning programs are available free via a download. Always backup important data first.
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