I don’t know why, but on my Windows 7, my wifi stops working after about 10-30 mins, and the computer goes very slow! I don’t know what that is, but I think it’s a virus.
Note: I have dual-booted Win7 with Ubuntu 12, and this only happens on Windows
Probably a issue with the driver of your wifi adapter. Update to the most recent driver from your manufacturer’s website. If there is no newer driver, reinstall the current one by deleting the wifi adapter via device manager letting windows reinstall it after a reboot.