After recently upgrading to Plusnet Fiber optic plus I was hugley dissapointed not to recieve the full speeds that I had been promised

To cut a long story short I discovered that I could only recieve speeds of over 30mbps on a pc using windows7 and on my main pc which uses XP the speed seems to be capped at 16mbps.
I would be hugely grateful if someone could suggest how I can adjust my lan setting to fix this problem (if indeed that is the cause)
Thanks in advance
