Locking your pc?
 

I'm trying to find a way of securing files and folders from a 'guest' when I'm away from home... any ideas?

create a guest account for your computer.

build your system into a server case and lock it with the key.

Hide the power cords or you could set a password in your CMOS this way they get a log in screen as soon as they try to boot up :)

It's a home system I'm trying to secure access to - it's running XP - is there any way to prevent file access from other users? A program or something?

you still have CMOS, dude. Password it.