I'm surprised nobody touched on this although some gave you excellent advice, excluding the illegal posts. (O:=
Helping users at home can get you fired, even if they is not a policy against it. How? If something goes wrong, the user can cause a problem at work or even claim the company is responsible. If anything is true, you can sue anybody for anything.
As suggested, use ONLY their legal software. Ignorance is no excuse especially when you have to click I AGREE to the license agreement during installation. YOU installed it. YOU can be held liable. How long do you think it will take before the user rats you out? Time doesn't travel that fast. If they like your work they can also cause you grief by telling others what you did. "HEY JOE INSTALLED XP PRO ON MY PC FOR FREE!!" What is the best form of advertisement? Yes, word of mouth.
Do it on your time. Never answer personal issues at work. Never let them bring their PC to you at work unless you collect it in the parking lot.
Write up a document, that an attorney should review, may cost you $150, stating your agreement with your customers. Make sure it states your current employer is not associated and they will file no judgments against them. Your attorney will have better language.
Unless you're an LLC or corporation, your personal assets are vulnerable should someone not be happy and decide to file on you. If you offer a warranty for your work, if you don't, if you offer protection in case of hardware failures, etc., state so in your agreement. Get a signed agreement before you touch any equipment. It's not personal, it's business. You're charging for your services. That classifies you as a professional. A written agreement is much easier to prove than an oral one.
And a final suggestion, discuss it with your employer. If you signed an agreement regarding not working outside work or with employees outside work, then you're in violation if you do. In Texas, where I live, it's illegal for them to restrict what I can do outside work. Texas is a right to work state. However, if I sign an agreement, then they can terminate me but the law will never let them write their own law so they could never do anything to me legally.
If you do not have an agreement you signed at work then discussing it with them shows respect. Also, letting them know you have an agreement which is required before any work is done to protect them will put them at ease. The only concern is that they may wonder if it will hinder your work for them. While they could not force you to quit doing it, they could terminate you for inadequate work. They don't need to specify why your work was inadequate.
All that said, I have been consulting outside work since 1987. All of my employers know that I do it and only one tried to strong arm me claiming I was competing with them. They were a VAR. I claimed I do not perform the any similar activities to any of their customers. I do not work on their time. I always ask new customers if they're currently doing business with you. We discussed it for several hours and I walked out with a raise to try to deter my extra curricular activities but I continued. I was only programming at the time and this company didn't offer programming services except for Revelation Rev. G (database development) and only in the Accounting arena. I did not develop in Revelation nor did I consult any accounting services. Nothing more was said.
It is not necessary to inform your current employer but since you're working for their employees, I suggest doing so. It's a lot better than them getting a surprise when someone comes to work and starts talking, good or bad. Reassuring someone you're not using their resources goes a long way.
Good luck!