Windows 7 without its respective updates is rather vulnerable. However, as long as you don't put any personal information on the computer before installing the updates, you won't be at risk of losing it.
Also, antivirus programs can be installed before you install the updates. I personally recommend Avast Antivirus Free. In the installer menu, I usually only install Online Protection, File Protection, Mail Protection and the updater. Anything else is unneeded unless you plan to put personal or financial information on the computer.