The distributed.net client is fairly easy to configure. To do this, after you first run the client, exit it and then go to your dnetc.ini file (in the same directory as the client itself). About the only thing below that needs to be changed in the .ini file is the e-mail address. If you are running the client on a slow machine, you might want to use a smaller fetch-workunit-threshold=[around 1000 or so] and a preferred-blocksize=30 instead of 33.

[parameters]
id=macguy@optonline.net
[display]
progress-indicator=rate
[rc5]
fetch-workunit-threshold=1500
preferred-blocksize=33
[misc]
project-priority=DES=0,CSC=0,OGR=0,RC5
[networking]
autofindkeyserver=no
keyserver=crosstalk.owns.com
[buffers]
checkpoint-filename=checkpoint
frequent-threshold-checks=3
[logging]
log-file=logfile
log-file-type="no limit"

Additionally, on a unix box, you can add -config to the pathname of the client and you'll get a slightly less intimidating configuration screen to look at.
More Info can be found on page 2