XP has no such partition on a clean install so you'll have to figure out what added this feature. It's not one from Microsoft.
I recently installed XP on an old laptop with a 250gb HD.
Installation is smooth except for one point. It keeps setting the "system reserved partition" at 100gb and effectively chewing up half the space of the HD.
Now I know this can't be right given Windows 7 only uses 100mb for this particular partition.
With that said I have Amobie Partition Assistant which will let me size this partition. Question I have is what is at minimum the size this partition should be set at without effecting the overall performance of XP.