If this is something that the company uses to keep their business up and running, you cannot use an off the shelf, home user edition of the OS type of computer. You need a server that can withstand some great lengths of time with the proper maintenance. I have servers that are still running that I purchased used in the 1990's. But I maintain them well.

You have not told anything about the error that you are receiving. You can always go forward in your version of SQL Server. But never backwards. Meaning, if your backup is of 2005, then you can restore that on a 2008 version. There is not need for you to be using SQL 2012. You are actually not using any of the latest aspects of 2012. You will not have a log in the backup file. You would have had to setup Log backups.

Sounds like you need to create a database of the the same name on that instance. And then restore your latest full backup and allow it to over write. You need to create the same login with the same credentials. What are the credentials that the front-end uses?

This is only a guess, since you did not tell us the actual error message you are receiving.

Andrew SQLDBA