I was told on the HP support forum that this 100 MB partition is likely associated with the Win7 licence, and if I remove it then my mother loses her win7 licence. Bizarre. Thread is here: Partitioning in HP P6510F is for what purpose ? - HP Support Forum
I also learned from same forum that I likely need to reinstall W7 with the AHCI settings: How to change Win7 from RAID to AHCI on HP P6510F desktop ? - HP Support Forum