When you open the Backup and Restore dialog, the Backup Path defaults to whatever was last used. Since you got a new computer, it's likely that the specific path doesn't exist on the new computer.
Do this:
1. On the Backup dialog in the first field where you are to specifiy the Backup Path, click the button at the end of the edit box and then in the resulting dialog navigate to and select the specific existing folder where you want the backup file to be created.
2. Accept the default value for the File Name and just click the Start button.