There are a couple of ways you can get the SA password, the easiest is to log into SQL (which will more than likely help you after you get back into SQL):

• 1. Login into SQL server using Windows Authentication.
• 2. In Object Explorer, open Security folder, open Logins folder. Right Click on sa account and go to Properties.
• 3. Type a new SQL sa password, and confirm it. Click OK to finish.

The other way that you can try is: 

Start a command prompt by typing Start – Run – cmd

Enter the following commands, pressing Enter after each line

OSQL -S yourservername -E

    1> EXEC sp_password NULL, ‘yourpassword’, ‘sa’

    2> GO

Where yourservername is, the name of your server and yourpassword is the new sa account password.  Type exit twice to return to the server desktop.

*Note for a Windows 2008 server the command is sqlcmd not Osql I believe

I'm running into the same problems this guy is having.  Changing the password didn't work either way.  I'm running Windows 7 with SQL Server 2008 and I've tried manually and run into errors when launching the local host.  Otherwise, I've been trying the installer and it keeps saying I need to use the "sa" account.  I can't get this to work out for the life of me, but would greatly appreciate any other help.