WindowsXP shouldn't have any troubles with a USB card. I just wanted to make sure you didn't have something like Win95/98/ME. While they do support USB, the support isn't as good or robust as in WinXP.
Do you have the manuals for your Dell? The Dell manuals that I have at home are pretty good at explaining how to install a PCI card. Dell's cases are some of the easier ones to open up too. Is the case one of the beige coloured ones (older), or a black/dark grey colour (newer)?
goates