I understand that you may need to run an older version of an OS for this, but does that PC require Internet connectivity? If so, what can be done to protect it?
Well, if you cannot update the browser or upgrade the OS (even a no cost Linux one), then the best thing to do is make regular full drive backups & use only when needed. Backups are the best physical restore point to good working order one can have, and perhaps the best protection. If these were available for Vista, my recommendation would be to use a VPN service (often on promo for as little as $2.99/month when purchasing & paying for 3 years service). Unfortunately, most of the best requires Windows 7 SP1 or newer, you'd likely be left with bottom of the barrel choices.
Yet you can still lock down the router for best Firewall defense, which I covered above. This is an task that way too many overlooks in their security configuration & provides much more protection than the one provided by the OS or a AV/IS package. If you're going to use this (or any) computer on the Internet, even if just a little, lock down the Firewall as much as possible. Instructions for a given model can be found with the instruction or setup guide, or be found online. Even the latest OS's benefits from this, security begins at the point of entry, not the Ethernet or wireless interface of any connected computers.
I still have a single XP Pro SP3 system, used only to assist a couple of close friends & when needed, it's still very useful Hyperterminal is a powerful tool. Which didn't ship with Vista onwards, not too long back, rescued a 500GB Seagate 7200.11 HDD, the infamous one that led an outright uproar across many tech forums 8-10 years back. Don't know what the big deal was, fixed mine in under 45 minutes with a sub-$15 gadget & had never done it prior. Point being, these OS's can still be useful when used offline, many of today's OS's doesn't have the tools of the legacy.
BTW, my uncle was an avid ham radio operator, this was long before computers were available (let alone affordable) for Home users. He was on that equipment a lot, don't know what exactly he was doing, only that could communicate for very long distances & had to have an FCC license. Other than that, don't know any other details.
Am sure that Vista has similar uses & it's 100% OK to use these legacy systems offline. Many machine shops are still running W2K to power the equipment used for machinery operations (example, the rebuilding of both diesel & gas engines). There's big money in this & I'm positive that the shop owners would be thrilled to upgrade to a later OS (actually W2K was/is more secure than XP), it's just that the OEM of the equipment didn't rewrite software to work with newer ones. Rather were focused more on manufacturing equipment that works with the latest OS (be it Windows or Linux) available.
Good Luck with the computer & your ham radio hobby, don't believe as many uses these as long ago, first time heard it mentioned for years.