I have Windows Vista and my laptop won't access the internet although it does connect to the wireless network. My iPod touch can access the internet via the same wireless network. Why might this happen?
Since you can connect but not surf, I suspect the issue is coming from your AV suite or your firewall software.
To test this theory, disable them both temporarily and try to get online. If you can then surf, one of the apps will most likely need to be configured or updated.
Anecdotally, I have seen many machines unable to get online due to an expired trial version of an AV program that came pre-installed and that the user was unaware existed. Uninstall any AV program/s you are not using.
If these steps don't help, try pinging the router and then a name or IP on the Internet. Maybe you have issues with your wifi adapter.