Can my employer force me to fix employee's personal computers?
Short version - probably, yes.
Most job descriptions, if they're actually written down, include a line similar to "other duties as assigned". If you don't have a written job description, your duties are likely "whatever your boss assigns you". If your boss wants you to do this, you should, because otherwise he's likely to walk away thinking "I gave him a task to do, and he told me no".
However, you could do a couple of things.
1) When these requests are made to you, you could check with your boss to verify the order in which he wants you do to things.
Hey boss, the team is working on the server upgrades we were talking about, but now Johnny sent me his laptop to repair. Is it okay if we finish the server upgrades today and work on the laptop tomorrow, even though Johnny's email says it "must be resolved by the weekend"?
2) As @user1108 points out, you could keep a tally of how often you're spending time working on non-work devices, and keep your boss up to date on it.
Hey boss, this week the team got 12 device repair requests, and they took up about half our time working through them. Is there a level at which I should push back and tell people we don't have time available to work on any non-work devices?
3) You could get permission from your boss for a new workflow, such as your coworkers having to get his prior approval before you work on any non-work devices.
Hey boss, is it okay if I tell people that you need to approve all non-work device repair requests before we can work on them? They're starting to pile up and I want to make sure you're aware of everything I'm working on before we spend any time on them.
And to your coworkers:
Hey Johnny, I just got your email, but requests for repairs of personal devices now have to be approved by [Boss]; I can't accept requests directly from you anymore.
Since all of IT is salary [...] they are not being paid for staying late and fixing people's personal systems, phones, tablets, etc.
You should definitely address this as well. The issue is that you're being paid a specific salary for a specific work week, and you're having to run past that to get everything done now that the scope of your duties has increased. This conversation can be presented from the standpoint of deserving a proper work-life balance.
Boss, I wanted to mention something about all the personal devices we've been asked to fix recently. It's reached a point where it's cutting into the amount of time we've got to complete our normal duties, so we're having to work extra time to get everything done. This really isn't sustainable in the long term - I don't feel like I can continue to ask my team to work so many additional hours. I think there are several different options, and I wanted your opinion on the best solution. Some of the ideas I had were to start turning down some of these device repairs, or to choose some of our other duties to eliminate or reduce, or to hire additional personnel to handle the additional workload. There may be other solutions as well, but I was hoping we could get clarity on this quickly.
The benefit to this approach is that you're not just coming to him with problems, but also solutions.
The worst-case scenario I see is that your boss doesn't see unpaid overtime / extended hours as a problem, and your team being upset at the additional work and causing them to consider looking for outside employment. Your best bet in that case is to let your boss know that your team is likely to be unhappy. Strictly from a hiring standpoint, you're unlikely to attract the best workers if you tell them during the interview that you work long hours for no overtime, so if one of your employees leaves, you may have a difficult time replacing them.