Hi Marco, I have XP as well and it works:
1. Take any mp3 file, use any editor (lots out there, just search) to edit it down to 30 seconds or less and get it so it sounds the way you like it.
2. Add it to iTunes library (drag into library).
3. Now right-click, convert to AAC
4. Right-click on the new AAC version, choose the Get Info option if you don't know where the file is (Get Info tells you the path to the file)
5. Remove the file from the iTunes library (but don't delete it from the computer)
6. Go to the AAC file, rename the .m4a extension to be .m4r
7. Double-clicking on the renamed m4r file should add it to iTunes in the ring-tones section and the checkbox should be checked (in case syncing is set to only be for checked files)
8. Check your iPhone settings on the Ringtones tab. You should be able to see the ringtones there - select them if you do not have the "All ringtones" option.
9. Sync the phone
10. Go to iPhone, Settings, and you should in the ringtones area see a custom area with your ringtone. If not, you may need to remove the files from iTunes, sync, add them again, sync. (It seems to fail if you at all leave the original files in place while also adding the ringtones)
Hopefully this works for you. It did for me.