Does Windows XP have an AGP slot for video cards?

WinXP has nothing to do with what kind of video card you can use - other than possibly having a driver for it. Your motherboard is the determining factor. The best at this point is PCI-E followed by AGP, then PCI then local bus (remember those - in 486s) and finally ISA. Of course you could also have an ancient IBM using a different format. Open the case and take a look at it. Get the name of the motherboard then Google it and you should get the specs.