Homegroup Created Outside Household on Secured Wireless

Recently I've been having abyssmal internet speeds (like .1mb/s down when I am used to haven't at least 3-4 mb/s down). This is when I noticed that a homegroup was created on my network by a PC that is not associated with anything in my household. My wireless has always been secured with WPA-TKIP, but I guess someone has been able to get into my network and create a homegroup on it. So I obviously can't remove the homegroup since I don't have the homegroup password or access to the PC that created it (since it's kind of obvious some one has started stealing bandwidth recently).

I went ahead and changed all of my router wireless security settings, but my concern is that my network can still be accessed through the homegroup that the intruder created even without the new keys. This PC is not showing on my network, however it isn't all that difficult to hide yourself. I would also like to know if there's any way for me to get this homegroup off my network completely since I may want to use the homegroup functionality at some point down the line.

I have a linksys WRT54G router with up to date firmware currently and I'm running Windows 7 Pro 64-bit on this machine (but I also have multiple other computers running both windows 7 and vista on this network). Any help would be much appreciated. Thanks.

You can delete the network via the "network and sharing centre" on all pc's (in control panel) (vista similar) you can also enable passwords protect in advanced settings - on the top left side - click change advanced sharing settings - click connect or disconnect and then create new homegroup with password protect.
Probably the best group of settings can be found on the network and sharing page on the bottom left side - click homegroup and most settings you need will appear including the advanced settings.