If you are on DSL you probably use the Schlabbach driver to connect via your Ethernet card to the DSL modem as I did until yesterday.
Now the leading German computer magazine c`t has tested the most used DSL drivers and found out, that your DSL speed will break in as soon as your connection is under heavy load. That means if you are downloading AND uploading at the same time as with P2P (e. g. Kazaa).
The only "driver" which holds up your speeds is the new cFos v5.04. I will not explain the technical details here. But this driver measures your actual connection line data and then modifies the transmitting priority of some special packets according to the measurement results.
If you already use the pastpath option on DSL and want to really use the benefits of your fast ping, you should try this cFos "driver". Installation is very easy and you can keep your former Schlabbach driver as well. The trial period is 45 days.
If the trial period is too short for you: I found a patch (for Win 9x/ME and 2k/XP) in Kazaa. Search for "cFos"...
I personally don`t use all the other options (ISDN, firewall...) as I am not convinced of that. But the DSL driver is full working and speeds up Kazaa a lot. If you don`t use pastpath the advantage will be not so tremendously high.
I like it very much although I`m a Schlabbach fan.
Where to find the cFos driver: www.cfos.de/index2_e.htm