This is easy to do, with XP.
If you have any data you want to keep, transfer it first.
Just right click my computer > manage > disk management.
The old disk should appear in the right hand window, with the partitions. Right click on the furthest partition first and pick " delete partition". Then do the same for the first partition and do the same.
You will now have no partitions, if it just had two.
Then right click the un-allocated space for the hard drive. If the format partition option is no there, then create a new partition. Just pick the default ( 100%) space and format it in your choice f file system.