It depends. You minimally need to have an available PCI slot on your motherboard. Or if you have an available PCIe (PCI Express) slot, you could consider a PCIe wireless network adapter. But being a slimline model you have to be extra careful. Due to space restraints, it's always possible some add-on cards won’t fit. Most probably will, but it's always something you have to consider.
Since I don’t a hp slimline s7520n myself, I can’t be more specific, but perhaps if you took a picture of the inside of your PC and posted it (or a link to it hosted elsewhere), it might help.
Frankly, it may just be easier to install a USB wireless network adapter. Since its mounted externally, you avoid a lot of these issues. But that’s up to you.
Finally, make sure whichever wireless network adapter you use, it’s compatible w/ your OS (including whether it’s 32 bit or 64 bit).http://www.microsoft.com/windows/compatibility/windows-7/en-us/Search.aspx?type=Hardware&s=linksys%20wireless%20adapter