1. Navigate to the Steam folder in Windows Explorer.
2. Right click, select properties.
3. Select the "Security" tab.
4. In the "Group or user names" box there should be a "Users (<computer-name>\Users)" entry. If this doesn't exist, click "Edit..."
5. In the next window, select "Add..."
6. In the "Enter the object names to select (examples):" box, type "<computer-name>\Users" and click "Check Names" (if the previous window had an "Administrators" entry, they can get the machine name from there, otherwise they'll want to get it from their system properties).
7. Assuming the name is valid, click OK.
8. Make sure all the permissions are ticked for that user on the Permissions page and hit OK.
9. It should propagate those changes to all files and directories in the Steam folder; let that process finish.
10. Hopefully the game will now run.