OK, Over. Your service provider is your communications network, but the other guy having access to your machine is a concern. Right-click on My Computer. Select Properties. A window should open that has a tab called Remote. If either of the boxes in there are checked, uncheck them. This tells your computer not to allow 'foreign' computers to use it remotely.
If your house is networked with the other guy's (unlikely, but it might be), your machine won't consider his 'foreign'. When you look at your folders, can you see any other computers there? Right-click and disconnect them.
Go to My Computer again, this time right-click and choose Disconnect Network Drive. It should give you an error (no network drives.)
Reset your security software to defaults. Have a look in there, too, to see if you're networked. Clear any exceptions. Reschedule your regular full scan (should be at least once a week, btw, and not the quick scan.)
IMO it's unlikely your friend does anything other than fix errors for you, but you never can tell. Disallowing remote should prevent it; the other measures are for double-checking. As Jodie says, it will be lovely to have control of your own life!