By "fails", I mean that it's still connected to the network, but I cannot access anything. The Internet does not work, I can't access the router page and I can't even ping any device on the network.
I have a Lenovo T61 with Windows XP. The wireless router is a Belkin N. I haven't noticed any cause for this (e.g. heavy downloading etc.) except that it usually happens around 9AM (weird, I know and probably irrelevant). Also, restarting the wireless connection does not work and restarting the computer works most of the time.
The network is fine since I have other wireless devices on it. I've only tried a wired connection to the router when the wifi failed once, and it might have worked.
UPDATE: Went through the router page. No IPs or MAC addresses are blocked (Belkin N can block devices during some time of the day).