Only one way to find out: close all other programs, disable your memory-resident programs like MSN and your pretty cursors, if you have a big pic as your desktop background, turn it off. And then see whether it will let you go past 103 layers.
I just checked on mine (PSP 7.04), and it only lets me go up to 100 layers (background+99). So your 103 is strange. Probably it's just time for both of us to move up to Photoshop