The problem is that Windows 7 won't receive security updates after January 2020. You can still boot to Windows 7 and use it normally; if you have an antivirus and you stay away from suspicious websites, it's likely that you won't get a virus.
The problem is that in the future, W7 won't be compatible with newer hardware (except for HDD/SDDs and optical drives).
TL,DR: Anyway, if you use that machine without being connected to the internet (disconnect Ethernet cable AND turn off WiFi), you will be fine.