Just from read your next input sounds like your computer has been infected with something. If you don't use anti-virus software, get some. I suggest AVG, download,install,update,scan fully. Then if you don't have spybot search and destory, download,install,update,immunize,scan. If those searches come up empty handed and you still have your problem. Backup what you need, zero wipe the drive, and reinstall windows,and your programs.
As for your wireless network, go back to manuals and read up on WEP encryption and not broadcasting for SSID. Change the name of your router and the password for it. Use at least a 18-24 character password for it. Change the default DCHP max distributed numbers to the very limit of how many computers use the wired/wireless connection. (take into account all wired and wireless computers, PS3,360,wii,etc) Change the subnet the DHCP gives out.(default is 192.168.1.100, change this start point to say 192.168.1.32)