BT Broadband as in Cable? If the answer is yes, don`t worry, that´s normal with cable, heh! if you were running winXP Pro then there is another thing thing called "Quality of Service" wich takes up, or reserves, 20% of bandwidth for itself.
BT broadband (and narrowband, for that matter) is known to have pretty awful reliability, speed and spiking. I've thought my computer was malfunctioning a few times before, and it turned out just to be BT messing up again.
Could also be that broadband takeup on your phone exchange has just increased. If a few people on your exchange are downloading massive amounts when they get home from work or school, this will have an impact on your evening ping rate.
If I were to put money on it, I'd say the fact that you'd just installed WinXP when this started to happen is nothing but coincidence. But just to be sure, try installing <A HREF="http://www.netlimiter.com/" target="_new">netlimiter</A> to keep an eye on the bandwidth your programs are using.
You need to uninstall the QoS Packet Scheduler or use a third party software or modify the registry to remove the 20% reservement if this does not help. And also don't forget to disable the QoS feature in your advanced settings for your ethernet adapter.