Graham,
Most cards work without any probelms, so, A few questions.
Can you tell me what type and model of soundcard you're using?
Did you load its software package or are you using the embedded Windows XP software to operate it?
Are you using line-in and line-out of the soundcard with stereo adaptors and looping one channel (i.e. right channel)?
Can you post the mixer settings for both record and playback like shown below? and if you are using the soundcards software, please post a picture of its settings...
PLAYBACK
playback.jpg
RECORD
record.jpg
brucek