I am using WINDOWS XP (service pack 2). My problem is that the playing audio files will lag while copying any thing

I recommend as follows:
1/
Look at Task Manager, double click the CPU column in Processes, and report what processes are using how much CPU.

2/
Perhaps you notice the disk making more noise than usual. This could mean disk thrashing during the copy/play operations.

3/
If you don't have much RAM, then the disk could be thrashing as it pages Windows in/out.

What's your hardware?

To find out much RAM and CPU you have go into COntrol panel -> System

its under computer:

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