I have a second hand computer Dell which has it's hard drive split in 2 parts - how do I rejoin this as the default C: is full and the other has still 9.44GB space left - if I try to download anything it says I have no free space

If there are no system or recovery files on the partition in question you might be able to delete it.
In doing so you will LOSE ALL DATA on that partition.
Open disk management: Start/Run/ diskmgmt.msc
Right Click the partition & see if you have a delete option.
If there are system or recovery files you will not be able to do anything from within XP.
You would need a third party program or else reinstall XP.
If this is a Dell installed XP (OEM) even reinstalling might not work as Dell may have mandated 2 partitions.
There is a good chance that there are recovery files on the 2nd partition.

All the partition software that I am familiar with costs a bit: $40+
http://www.google.com/search?q=parti...en&sa=N&tab=gw
I know of this one freeware if you are up to learning its' complexities:
http://www.ranish.com/part/

You could also send your downloads to the 2nd partition rather than C:

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