Pop/click/dropout is often a buffer size problem. Be sure to check your Buffer Size setting. You'll find it in the Edirol Icon in System Preferences or in the Control Panel of a Windows machine. Increase your buffer size until the problem goes away.
There are also several settings you will want to make in order for audio applications on a Windows XP computer run optimally.
One of the more detailed websites to walk through these settings is MusicXP.net.