Limewire seems to work the best. Don't opt to start it automatically, don't have it scan your computer for files to share, disable ultrakeeper abilities. disable chat, don't share with i-tunes, don't allow partial sharing.
to have your computer running at a good speed have a gig of RAM, a 1.5Ghz or faster cpu, and at least a 7200 rpm HD with lots of space. Also if your computer is running slow it may have other things starting at sysstart. Learn how to use "msconfig" and shut down what you don't need.
Use a firewall like Zone Alarm (free). Don't get all the extra trial progz they try to get to you to download. These will only slow down your computer and nag you to buy them.
Get a gateway/router with a NAT firewall (if you are using broadband). Linksys routers seem to be the best for the money.
To avoid comunication with the RIAA move your downloaded music and movies to another folder on your computer as soon as they are done downloading, and don't download the most popular titles(there is a reason so many 12 year olds learn about the RIAA).
when you are done downloading right click the LimeWire systray icon and select exit to completely shut down LimeWire. If you don't it will continue to use system resources that will prevent your computer from running at optimum speeds.
and the best way to not bog down your computer and maintain maximum security, use the old sneaker network. Get a flash drive and share with your firends without using the internet.