I doubt the sound card is the problem, but it would not hurt to pull the SB card out. Why have it in there if its not being used? I would think more interms of memory, or low power, and maybe a heat problem causing the problem.
You can download Memtest86, put the program to a floppy or CD and test your memory. Any errors at all usually means this is the problem.http://www.memtest86.com/
If your memory is good the next thing i would check is the 12 volt line for the power supply. I download Speedfan for this and it just so happens it also reads the motherboard sensor temps. If your temps are over 60°, or your 12 volt line is below 11.8 volts, this is where your problem is. You could also have a low 5 volt line but this is seldom the real problem as most of the time its both the 12 volt and 5 volt.http://www.almico.com/speedfan.php
If it is none of these it is usually a program running in the background using a lot of memory. In XP a ctrl-alt-delete brings up the task manager and look in the processes tab for programs using a lot of memory. Also look in the performance tab for cpu usage. You may have to go thru and find what program is causing this by trial and error.