There is a lot of malware out there, that will screw with the OS, and can do things like stop programs from running. That doesn't necessarily mean someone is actively in your system playing around. Running malware scans, such as the guide in my signature, can remove malware such as that. If you don't care about losing data, you can just wipe the system, and that will also do the job.
As mentioned above, you can add a router between your modem and PC. Then you can do things like hide the SID, enable encryption, and use MAC filtering. Your router would then assign an IP address to your PC.