XP auto-detects most PCI cards, including PCI sound cards. However, if
often ignores ISA cards, which were common on PCs before 2000 or so.
However, it can function with them if you obtain and install XP-compatible
drivers. You need to get those from the sound card maker. But, whether
they chose to make a set for XP is another matter.
If the sound is on the motherboard (a chip, not a card), then chekc with the
motherboard maker for drivers.
If a major brand, and on the motherboard, check with the PC maker (e.g.,
Dell, Gateway, Compaq).
If nothing else works, get a new, XP-compatible sound card.