1. Client Download
Go here and find the appropriate OS version of the command line client. For any Windows OS, the one you are looking for is called i386-winnt-cmdline.exe. Thats it for the client download. (What could be easier?!!)
I only recommend the command line client as it is typically at least 50% faster then the screensaver version of the same thing, so it speaks for itself.

2. SETIstash download
SETIStash is potentially the greatest gift ever given to a SETI user. Lately, with the increasing popularity of SETI, the servers which manage workunits have been flooded, and the bandwidth on which these servers reside has copped a hammering. The end result is that it can sometimes take hours or even days to get a new unit, once your current unit is complete.
Enter SETIStash, a program which allows you to download multiple work units all at once, allowing you to process multiple units without having to connect to the Internet after each one is completed.
You can download SETIstash from here.

3. SETIstash Installation/Configuration
This site also provides comprehensive instructions on how to use it. I have borrowed these and modified them without permission of the owner and posted whats left below.

1. Extract the contents of the ZIP file into an empty directory where you want to run SetiStash. C:\Program Files\SetiStash is a good choice.
2. Rename the file you downloaded above as seti.exe and copy seti.exe into the SetiStash directory you created in step 1.
3. Connect to the Internet.
4. Open the SetiStash directory with Windows Explorer and double-click the login.bat file. This will launch the CLI and ask you to either login with your SETI@Home user ID or create a new account. After you finish logging-in to the SETI@Home server, login.bat will initialize each cache directory with your user information file
5. Run SetiStash.exe to launch SETIstash. You should see 10 EMPTY caches.
6. Click the TRANSFER WU's button. This will begin filling the empty caches until they all read NEW.
7. When at least one cache reads NEW, you can click the PROCESS WU's button. You will see the command line go to work and SETI will begin processing.

Once you begin seeing caches listed as COMPLETE Xhrs Xmin, you can hit the TRANSFER WU's button, and SETIstash will upload the completed unit and download a new one. Refer to the image below to understand the look of SETIstash's main window.




Left you can see the SETIstash window, with a completed work unit, work unit in progress and several unprocessed work units. You can configure the number of offline units SETIstash will store for you by clicking the SETUP button and increasing the number of caches. Doing this will increase the amount of time it takes for your machine to process all work units and is a good idea for fast machines, or machines that cannot be on the internet all the time.

4. Joining Team AtomicMPC
This will get your name on the Atomic team battle roster, and as such, all results you process will be added both to your personal totals and the team total. Go here and click the join link in the top left hand corner of the page. You will need to enter your email address and password here. Within the next day or so, your name will appear in the table.
Congratulations, you are now a part of the team!!!