Really bad experience
I recently installed the demo version of Internet Security on my WinXP SP3 PC.
First of all - and I really dislike it - I was forced into de-install my current AV (Kaspesrky), while disabling it would have been enough.
Then, after the installation of Internet Security, Windows behaviour became erratic. It starts to complain about system files being replaced by unknown versions. Some customisation of my PC disappeared (such as the replacement of the Task Manager by Sysinternal-ProcessExplorer, ...)
With so many problems, I just erased my C: and restore my previous backup.
Very bad experience with F-Secure and wasted time.