Hello, I have recently got the same Fujitsu Stylistic model, and I was also having troubles installing the pen. But finally it started to work, so I can try to give you my description.
First, I installed Windows XP Pro Service Pack 2 (not the Tablet Edition, because I don't have original recovery cd). After installation pen of course didn't work, graphics had poor performance (default driver), tablet buttons didn't work, but the sound worked and probably the wired network too (I didn't check, but it looked like the driver is installed). In device manager I had two Unknown Devices (which turned later to be the pen and the tablet buttons - details later), an Unknows Network card, and also as far as I remember an Unknown PCI modem.
Then I downloaded all the drivers for ST4120 from http://www.pc-ap.fujitsu.com/support/dr ... 20_21.html
site (but they didn't work - details later). First of all I installed the video driver (st4120_21_video.zip) and it worked fine (appropriate video driver seems to be required for the pen to work). Next I tried the pen driver (st4120_21_digitiz.zip) but it didn't work, it didn't even recognize any device. Now I think perhaps this is driver for the active tablet version (with battery powered pen?), and ours ST4120P (P is probably for passive) probably requires another driver.
Fortunately, I have also found http://support.ts.fujitsu.com/com/support/index.html
site. Here you can find drivers specifically for the ST4120P version, not just ST4120. I tried with the pen driver, but it also didn't work at the begining. But then I've done something like that (unfortunately I don't remember perfectly): I selected both Unknown Devices in device manager and uninstalled them. Then, I clicked Add new device in Windows Control Panel, selected 'add new device' or something like that. When the wizard asked me for the device type, I clicked 'have disk' and pointed it to the path, where I have extracted files for the pen driver (downloaded from support.ts.fujitsu.com site). Later I have installed the tablet buttons driver in the same way. After this (and perhaps a reboot, I don't remember) the pen finally started to work !
Hope this will help, if you have any questions just ask me here.