Help! Need A "To Do" List: Someone Gained Acesss To My Wireless Network

[blueshift]

I saw a Device On My Network That Was An Unnown Device. I have encryption enabled. I would like recommendations to assure security.

How I Found Out: My Internet started becoming slow. I have 40mb down and 4mb up, Ran Speed test noticed My down was limited to around 4mb. So I checked to see what devices were connected and there was one I was not familiar with. I turned off every wireles card just to make sure and it still showed up when refreshed. So I decided to

1. rename ssid
2. change encryption pass
3. change router pass
4. enable mac filtering/forwarding

All the windows computers have firewall enabled and I disable any netmeeting or remote acess stuff in the services.
Should I do something else? I know if the person was good enough they could of accessed my computers and accounts.

Do you think I should change all my internet passwords?
Would it be extreme to format all the hard drives?

I get a weird feeling because if you wanted to use internet or download stuff, why not use a open network?

Any help would be appreciated.

[Geek-9pm]

How many home computers on your wireless network?
What brand of router do you have?

You should already know , bot routers and Windows have cache. Even after something is disconnected, it may still appear in the cache. I am saying this for the benefit of others who read this.

At the command prompt:
ipconfig /release
(wait a bit)
ipconfig /renew
Also:
ipconfig /flushdns

The above will force Windows and the router to forget whatever was using the network. Check later to see of the unknown devices come back on.
If they do, then you have a significant problem.

Have you done Anti Virus scans?

What other equipment do you have that uses the Wireless?
Printers, TV cast dongles and other gadgets might show as unknown devices.

Here is a recent article that applies to Windows 8 and 8.1 PCs.
http://www.eightforums.com/system-security/50657-unknown-network-devices-appearing-despite-firewalls.html
Unknown network devices appearing despite firewalls

Disclaimer: This is for informational use. Do NOT download the software that  they offer. Unless somebody else on this forum can  confirm the validity of this link.
 

[DaveLembke]

If you can identify the MAC address of this unknown device you can find out pretty quickly what it is.

I ran into a similar situation last year about this time with unknown device. I went crazy trying to find out what it was. I then decided to tackle the issue smarter and figure out the MAC address because the first 6 hex characters are a manufacturer ID for the network adapter ( unless spoofed by a hacker ) and you can look up the hex to see the manufacturer.

In my case the unknown device was found to be a Kindle Fire. AND more interesting of all was that when the Kindle was powered off, it was still connected to WIFI ! 

http://www.coffer.com/mac_find/