RC5, the way we're talking about it here in the DC Forum, is Team Anandtech's effort to decrypt an encrypted message. The message was encrypted by RSA Security Labs, who published the first words of the message and set a public challenge. Anyone who could decrypt the message wins a cash prize. Most of us are having fun with the competition aspect of this contest, not focusing on the money part, but there is the possibility that one of us will win $1000! Team Anandtech is overtaking the #1 overall team in the RC5-64 Challenge (namely Team SlashDot). The Dutch Power Cows are overtaking both Team Anandtech and SlashDot right now but we have a big lead on them.
The way that you participate in this particular contest is to download the client program from Distributed.net and then let it run in the Taskbar (assuming you run Windows). If CPU cycles are unwanted by any other program, the client will use them to test possible 64-bit keys to the encryption. It needs to connect to the Internet at least once in a while to send in the keys it's checked, and to fetch more possible keys. For computers that can't connect to the Internet, we "sneakernet" some keys on a floppy disk and fetch/flush that way.
The client program is really well-behaved, about the only time you'd need to turn it off is possibly when recording CD-R's.
See the link in my signature for more info, it links you to the FAQ. Everyone in this Forum is really nice, and I bet by the time I click the "Reply" button there will already be several other posts with info for ya... ; )