I'm on XP Home SP2, pretty un-modified install, just the critical/security updates.I also run Norton AV but and Black Ice Defender, but shut them down when installing new apps.
I already had a version of the Logitech profiler installed for my old USB gamepad. The new one installed straight over it, picked up the gamepad, added the G-25 to the list and it just worked,no calibration loss, or FFB problems. Although I can't remember whether or not it demanded a restart.
TBH I thought installing the software was easier than plugging in all of the bits and pieces. I'm wondering if people are describing a software conflict rather than a bug.