The software install is easy - just drop the CD in an dfollow the prompts - you don't need MSDE on the back office computer, so you can Skip that part of the install.
Here are the things you will need to deal with:
1) Firewalls - Win XP SP2 Firewall, or other "Personal Firewall" software on the register will prevent the back office machine from connecting to the database
2) Finding the 'Register' on the network - firewalls can again be an issue, but you can usually solve this by using a Static IP Address on the register.
3) You need to create a second Register - open SO Manager at the register. Go to Database/ Registers/ Register List. Create a new Register Entry - you should not configure any devices for this register, but you will probably need to go to the Receit Printer 1 & 2 tabs and clear the data before you can save the register. On the Back Office computer, in SO Administrator, go to File/Configuration and set the Register Number to 2 on the Register Tab
4) Setting up the database - On the back office computer, in SO Administrator, go to File/Configuration. On the Database tab, enter the same values you did for the Register. If the register says "(local)" for the Server name, you will need to use either the Windows machine mane of the register, or it's IP Address. Click the "Test connection" button - if you don't get a successful connection, then items 1 & 2 above are almost certainly the problem...