Hi. No expert but could be your broadband provider's settings. Or your Firewall if you have one. Assuming you have this via phoneline, have you got microfilters on all phone points? And tried the old 'switch it all off. And on again' number?
Suggest your provider's help page first of all and check all your settings.
Some laptops have a button (maybe above keyboard) that you have to press to enable wireless. Make sure this is on.
If thats not it, then it could be drivers. I had this problem with a laptop with windows vista and after trying several things, I eventually had to update various drivers on the laptop in order for it to use the wireless connection.
So using an ethernet cable you are able to use the internet...
Can you see any status pages that show 'connected devices' ? Your PC is connected, of course, so that should show up.
On the wireless side, were you prompted for the SSID (it's a form of 'beacon' and most routers have one set as default - around here there are 2x NETGEAR a Zoom and a couple of BT (but they have a few numbers added to make them unique, plus someone on Sky).
You should also have encryption set up, and have been prompted for that by the PC the first time you try to connect.
When connecting, it is common (though can be avoided) for the PC to ask the router for an IP address. That's commonly used for office environments where individual staff may hot desk and when they're in the office they just need an unused IP number for the time in the office that day. It depends on a service in the router called DHCP being on.
If you right-click on the wifi icon in the system tray (down by the clock) and can get 'status' then you may see an IP address of 192.168.xx.yy or 10.0.xx.yy (where xx and yy are allocated by the router).
You may need to click 'advanced' or 'support' tab to get to see the IP information.
See what IP is being allocated as that may help identify what's going on. At a guess, DHCP is on, else even using ethernet cable, it would not connect to let you use 'net.
Right, off to bed for me.
If DHCP is disabled, then some other IP will be set and there will be a message like 'limited connectivity' shown.