First order of business, pull up information on the lawyer that initiated contact with you to determine how much experience they have at the firm. If you're a small company they may have someone with limited experience, say three years, and if so, argue as much as possible and you may distract them from one of my other points.
Secondly, forget anything you believe to be true about software licensing and forget about license agreements included with software. What Microsoft, Autodesk, Adobe, etc. licensing department tell you on the phone and what they state in their licensing terms is not true and will not hold up legally unless you have more money than the fines to afford lawyers to fight the big guns. It's not a legitimate license unless you have a receipt. This is important, I repeat, you do not have a legitimate license unless you have a receipt for it. It doesn't matter if it's past 7 years, you have product keys on the side of your chassis, or you have discs; you must have it on the receipt.
Thirdly, do not provide information unless you're specifically asked for it. Read what they've requested, interpret it as literally as possible and if that allows you to include some information and not include other information. This point may not seem relevant to you and I'm not going to get into detail, but I want you to consider this point for at least an hour as the outcome may have a huge monetary difference.
Fourth, you can't buy stuff now and attempt to pass it off as something you'd purchased before they served you. Don't even consider back-buying software you didn't own before. Date of receipt ties into point number two.
Fifth, consider how they obtained this information and how much the person who provided it really knows. I won't give you advice on what to do with the software this person may not be aware of but I'd ensure your file servers are Linux and if you've ever made a transition from Windows to Linux, hopefully it was a transparent process to the users.
I won't get into details over our case as it cost us a tremendous amount of money, five figures, and at the same time, they may have missed a lot of stuff (the site is certainly fully legal now). If you have any other questions, feel free to fire them off and I'll try to answer as well as possible. The best advice I can give you is to consider this a logic problem.