How many connections are you allowing? The more connections you allow with BT, the less chance you will get to surf normally. It's like a miny DOS attack when you think about it. You have all these people sending or taking information off you that the pc has to allocate more and more cycles to, so when you want to do a bit of surfing, you are fighting all these other people to get out onto the net. XP makes it a lot easier (Win 98 was nigh on useless if you wanted to surf and torrent at the same time) but it can still be slow due to the amount of things XP runs compared to other Windows installs, which are all trying to get a piece of CPU ass
I gave up on torrents almost a year ago but if I recall rightly:
If you are using Azureus and not getting the yellow smiley face then dump your router. If you still have the green not so smiley face then you ain't got any remote connections and, basically, tough, but if you can find your IP address (not your proxy) then go into options and put your IP address in the bind your ip box - this should help you to get the yellow smileys and the better speeds - but not on Piratebays trackers. They are notorious (well they were) for not liking non-scandanavian IPs.
Try another client. Azureus, whilst probably the best overall in terms of design, style and support, suffers greatly being Java based and power hungry. It may only be using 30% CPU when you looked at it, but keep watching the beast - it'll shoot up and eat into your memory the more connections you allow.
Or better still - dump BT. P2P is a strange irony in this security conscious world. We have virus killers, pop up blockers, advert blockers, content filters, firewalls, routers and spyware blockers and destroyers, but we'll still allow 200 strangers access to our pcs in order to download a crappy cam of the latest blockbuster.