Something propably changed in your router configuration.
When you got internet working on your comp, start a command prompt and type
ipconfig /all
After you lose your connection, do it again and check your IP.
It may be come DHCP problem (if you're letting your router set your computers IP). You can try de-activating your DHCP option on your router, and set your computer's IP manually (to something like 192.168.1.2, or 192.168.0.10.. depending on your router's own IP inside your local network)
You can also check your router logs, if you can access one.
If this doesnt help, then warm up some good antivirus and learn using HiJackthis to clean your system.