The serial port driver will be part of the chipset drivers of which Windows should have, but in this case doesn't appear to, native support for. One thing you can do is if you don't use the serial port and assuming Dell hasn't hidden the option in the bios, is to go into the bios and disable it.
Looking at the list of available Vista downloads for that computer, Drivers & Downloads
in all likelihood the "PCI Simple Communications Controller" is a good old dial up modem (Dell must have a warehouse full of these things, because it looks like they are still including them even in their most recent computers). If that is what it is and you don't use it, the easiest and best solution is to open the case up and just remove it. If you do use it I wold go with the Vista driver for from the link above.