Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: I have a dell xps 8100, about 3 years old with 2.0 usb ports. I have three open slots for cards. Will this work?
Document: I have a Velocity Micro which is just old enough that it came with USB 2. I use several large USB 3 external drives. My computer is fast and stable. A couple of years ago I tried two different brands of USB 3 PCI cards (not Anker). The increase in speed compared to USB 2 was dramatic. With both cards however I was subject to frequent blue screens. Recently I decided to try USB 3 again and bought and installed  this Anker card. Third times a charm.
I have an old (4 years) tower. USB 2.0. I wanted to upgrade to USB 3.0 so I can get faster transfer speed to external hard drives for backing up my PC. Saw this card and purchased it. Installation into my tower was a breeze - found a slot and attached the power cord. Rebooted and was stepped through the driver installation. Took about 5 minutes for the install and then another reboot. Wow. This is a rocket compared to my 2.0 transfers.
I have a Dell XPS 8300 desktop. It's a fine machine and I have really gotten my money's worth from it. For external drive connectivity, I have used the eSata port on it and a 1.5TB drive. This has worked well for over a year.However, WD My Passport USB 3.0 drives are insanely affordable now and come in 1TB, 1.5TB, and 2TB sizes in the handy 2.5" format. I have used these many times at work and they are awesome.
After doing a ton of research on various USB 3.0 PCI-express cards, I finally settled on the Anker for a combination of price, positive reviews, and the fact that it uses a VIA chipset (versus a more generic chip.) I'm happy I went with this card and I have no complaints. Transfer speeds on my USB 3.0 hard drives went from ~30 MB/s on my existing USB 2 ports to over 100 MB/s when using this card.
I had no problems with this PCI-e to USB card at all. I installed the card and followed the instructions on the pamphlet and within minutes I had 3 new 3.0 usb ports up and running perfectly fine. I would say to anyone thinking of making this purchase, go ahead and get it. You be happy you did.
While I have built many Windows computers over the past 17 years, I love the way things are much easier now. I placed this in my Windows 7 (64bit ultra) computer (that has a 3 year old motherboard in it), booted, and there were no problems. Thanks for a great product.
I installed this into the PCI-E slot of my Dell desktop and it worked right off the bat. It provides a very noticeable speed boost to any USB 3.0 peripherals I plug into it. I use the internal USB port to connect to an internally mounted StarTech USB 3.0 front panel hub. So now I have 4 USB 3.0 ports accessible on the front of my desktop. Sweet.
I added this card to a Dell server that I wanted to attach an external USB 3.0 drive to. Installed no problem and even came with a sata to 4 pin molex connector. Drivers installed automatically.I also bought one for my desktop computer to add more USB 3.0 ports. I installed a USB 3.0 in the internal port and used it as a readyboost drive.Installation was simple and the card feels very solid.
You know backups? Because your hard drive is going to fail and you are going to lose all your pictures, and movies, and that thesis you have been working on for 2 years. This drive will be my connected backup, carbonite is my near online backup, and I also make quarterly dvds. Yeah, I'm not losing my data.My XPS 8300 does not have USB 3.0 BUT it does have a PCI-e 1x slot. Ok, let's go shopping.
Received card and unpackaged. Confirmed open slot on motherboard. Installed and powered on. Driver loaded automatically (Windows 8.1). Both my 2.0 and 3.0 USB drives connected and worked properly. Very happy.
Answer: It should probably work given that your PC motherboard has a PCI-e(PCI express) slot and it's not occupied. PCI-e slots are specialized slots and smaller in size and typically come as a single slot on motherboards, the free slots you mention are probably not PCI-e slots but PCI slots.