Old-School WoW only runs on client version 1.12.1, but you can have two versions of wow on the same computer.
First you need to apply for and account
Here in the Account application area.
Step 1: Load all 5 classic World of Warcraft cd's. If YOU do not OWN a copy of the WoW client then you'll have to Buy a copy from a Local store. We do not condone the pirating of illegal Copies on the Wow Client.
Step 2: Run World of Warcraft, log on with any acct info, and only accept the first patch to v1.12 then exit out of wow.
Step 3: Download Patch v1.12 to v1.12.1
HereStep 4: Run that patch then exit out of World of Warcraft again.
Step 5: Now you go to your world of warcraft folder and open realmlist.wtf in wordpad. Change "set realmlist us.logon.worldofwarcraft.com" to "set realmlist old-school.kicks-ass.org" without the "".
Step 6: Delete the normal world of warcraft shortcut from ur desk top.
Step 7: Create shortcut to WoW.exe in you World of warcraft folder and drag that to ur desktop.
Step 8: log on to WoW with your new shortcut to bypass launcher and avoid bothersome patchs and enjoy
Post and problems or questions in the Tech support forum