There is a way to install W2K on top of XP and I can probably pull out the information if you really want it, but experience suggests, a clean install of '2K is probably the better option.
The easy way, is to reformat your drive and select CD as the first boot device in the BIOS, pop the CD in and off you go. Exceptions, include a motherboard that has a RAID controller on. Make sure you read and understand the process for installing the drivers during the installation process.
Remember to back up any important information. Personal files, Address Books, Favorites file and the like, as you will loose everything.
Whether it will be faster depends very much on the specification of the computer. For pure speed on an older machine, you wont beat '98se or ME!