How can you tell if someone is using your IP address to log onto the Internet?

One IP address can only be used by one person at a time. Many ISPs give you a new IP address every time you connect to the internet, or give you a new one every day or two if you use a broadband connection. If you get a message about an IP conflict, restart your computer. Chances are you don't own your IP address, they rotate.

Whilst the above answer is partly correct you could have had your browser hijacked. This would allow a remote machine to use your system as a botnet sending out spam mail or worse and you do not know it. There are many free programs to download called "network sniffers" get one and it will tell you who is on your network. Your other problems are system security related. System security is never up to standard on most PC's simply because people do not know enough about it. I use AVG Free antivirus, it is good, but a bit of a memory hog. I use Zone Alarm free firewall and windows firewall is turned OFF. If you haven't got it, search for Spybot, download and install it, it is free. Make sure it is updated and scan your system, it will find and remove all sorts of nastys. When you have done that use it to immunise your system. Make sure teatimer installs. All programs have to go into the register and teatimer asks you to confirm every addition and deletion from the registry. With that running and if you have not just installed/deleted any software and teatimer asks you to confirm an entry, disallow the entry as it is surely some malware, stop all programs and scan with spybot and your antivirus. Now go to Trend Micro and download the free "Hijackthis" and run it. It gives you a list of all processes that have linked to your browser. If you do not recognise any entry, check and fix it. The worst thing that can happen is you may have to re-install a software package BUT you have ensured your browser is not hijacked. Then get a free package called "Hookanalyser" and install and run. Any entry shown in red is a rootkit infection and so check and fix it. If you use Zone Alarm, there are about 30 entries in red but they are all called "true vector services" and are a part of Zone Alarm, leave them there. Now you can be sure you have no infections. Lastly download "Abexo free registry cleaner" and install and run. Delete all it finds and your system will be clean and running at optimum speed. AVG updates automatically every time you boot up. Zone alarm tells you if an update is available and Spybot must be manually updated. AVG and Zone Alarm are best left alone once installed and running. Zone Alarm will keep asking you for permission for a program to be allowed access to the internet/trusted zone. If you check the box "remember this answer" it never asks you again, permission is permanent. Spybot should be run at least once a week. Hijack and Hookanalyser should be run once every 2 weeks. Good luck!

If you use broadband the only way to get a new ip is turning the modem off, Changing what the modem plugs into, or getting a new modem