There's not a lot you can do to fix the problem.
Your system is the equivilant of a 1Ghz Pentium III and you only have 256MB of RAM. It's a slow system by today's standard, and adding the external sound card requires additional CPU resources to drive the sound hardware.
You can try adding more RAM, but I don't see you'll see that much of a performance increase.
Short answer: That PC is basically an old, slow piece of poo.