You really should upgrade, there are quite a few known exploits already for Windows 7 and since it's EOL it won't receive any patches for them, but yes you should be able to update the cert store.
It's your system to risk. There are plenty of attacks that require no user interaction to exploit, and malware can run completely hidden from the user and even out dated AV such as rootkits. Hope you don't access financial or other confidential data on that system.