Even though Giga streams from hard disk, it still needs to load the beginning of each sample into ram - so there is a minimum amount of spare ram you need for each instrument you load. XP takes a certain amount of ram as well. Perhaps the full Gigapiano and XP require more than 256 free ram in combination.
The Gigapiano has a \'lite\' version on disk which doesn\'t use the release samples. Maybe that will work better - but you mustn\'t load the other versions as well, since even if you don\'t put them in a channel, loading them into the loaded instruments window is what uses the ram.
Use the quicksound window, find the Gigapiano and click on the + sign next to its name. This will expand the instrument contents list to show the \'sub instruments\'. Try loading just the \'lite\' version and see if that\'s better.
Alternatively, if you have loaded the piano and all the sub instruments, try unloading all but the lite version by right clicking on each one and chooseing \'unload selected\'.
Well, there aren\'t many other reasons I can think of for getting that message. Perhaps you can make more ram available by removing some of XP\'s operations. Unfortunately I don\'t know XP, so I don\'t know how possible this is.