Wireless Internet Connection Lost After A Few Seconds?

The laptop can see the wireless networks (there are two wireless networks since it's a 2.4GHz/5GHz dual band router) and even connect to any of the two networks.It also gets an IP address from the DHCP server and the Internet connection starts working for a very short period (5-10 seconds). Then the connection is lost and though it occasionally returns, the computer is unable to access Internet most of the time. Apparently, it doesn't even get to the gateway. If I ping to the router's IP address there's no response, except for during the first few seconds after establishing the connection:[CODE]This behavior is absolutely consistent and can be observed each time one connects to the Linksys router from this laptop. Nothing I have tr includes: rebooting the laptop, rebooting the router, disabling one of the two wireless networks on the router, trying out various encryption settings (open, wep, wpa2), changing channels, disabling/enabling b/g/n modes, disabling firewall on the laptop, disabling ESET antivirus, reinstalling the wireless adapter driver, ... I could go on. I've been struggling with this for three nights and I'm getting desperate.

There are three other laptops (one Mac, two Windows PCs) connecting to the router without any problems. Strangely enough, the laptop only has this problem with the new Linksys router. It can connect to Internet via our old Airport Express wifi router without any problems. It can also connect to Internet via the Linksys router with an ethernet cable.One thing I have discovered is that rebooting the laptop in a safe mode with network enabled FIXES THE PROBLEM! However, when I boot back to the normal mode (even when disabling all startup items and all services, except for those needed for networking), the problem is back.