The best way to find out is to look over any operating system information on the driver disk to see which versions of Windows are able to use the current ones. The manufacturer's support site would be the besy way to insure the latest drivers for 2000/XP are used. There you can go through the list of drivers to see which ones are currently available.
Not on all hardwares. You have to go to the support site to actually see what updates are available. What is the make and model as well as type of modem you are trying to get drivers for? For a dialup modem like a 3Com model XP has generic drivers already. It has a limited amount for others as well.