"You may experience random connectivity issues when you connect Windows7 or Windows Vista-based mobile PC to certain Wi-Fi "hot spots." These connectivity issues include following: wireless network connection is dropped, You experience poor performance. You experience these issues if computer is running on battery power. Note You may also experience these issues when you connect Win7or Vista-based mobile PC to a wireless access point (AP) in small office/home office (SOHO) environment or in an enterprise environment," explains Microsoft in KB928152.
To resolve: Method 1: Connect mobile PC to a power source – When you plug mobile PC into a power source, Vista or Win7 switches wireless network adapter power setting in default power plan from Medium Power Save setting to Maximum Performance setting. This turns off 802.11 power save mode.