I assume you are referencing the drivers for the FTDI chip of that board found here.
There are indeed two drivers for XP. As is the case will a lot of software one version is needed if you have a 32-bit operating system and another for a 64-bit operating system. The x64 version is the 64-bit version, the plane version here is the 32-bit version. (In other cases you may see it labeled x86) You need to download the one that matches the version of Windows XP that you are running.
To check that right-click on My Computer and select properties. There you should be able to see the specifications for your OS. The odds are that since you are running XP it will be 32-bit, but you should check.