Unfortunately some older hardware is not supported by newer operating systems. Some of it just is not compatible and some of it hardware vendors have decided not to support. A PC that was designed for Windows XP can be very old (in computer terms) and bottom line, PC vendors are in the business of selling new PC's, not prolonging forever support for older hardware.
How much do you want to spend for a sound card? or how do you want to use it? e.g. if you want 7.1 surround sound that type of card is going to cost more. Also, if you only have older PCI slots in the PC that will limit you somewhat on sound cards. Look over the offerings at www.newegg.com
I have an older PC that has an HT Omega "Striker 7.1" PCI type sound card. It works very good. Most of the new model SoundBlaster Xfi line is PCIe - another hardware obsolesence as the PCI bus has been replaced by PCIe. New motherboards that have PCI slots are not "native" PCI slots but just "bridged" from the PCIe bus and some PCI cards will not work in these "bridged" PCI slots.