I've just installed Win XP Pro on a Fujitsu/Siemens Laptop. BIOS says 500 MHz AMD K6-2+ but MyComputer/Properties says Processor 200 MHz. Do I need any specific AMD-drivers to get the real speed out of the processor?

That speed is because of PowerNow! Get the computer to do something serious and then check the speed again....

Make sure you have at least 256 MB of RAM, otherwise it will feel painfully slow running XP.