Why does my pc say it is low on resources?

I have a Dell, running Win 98 2nd edition, pent III processor, my C drive has 2 gb of free space (100 % free space) and right now it says my system resources are 64 % free. The problem is that after I am online for a while or playing games online it says that my pc is dangerously low on resources. Can anypone tell me why it is doing this and what do I need to do to fix it?