Most likely it's because Windows 8.1 trying to use its own network adaptor driver, which may not be compatible with the network adaptor in your system.
Try the following and it should fix your issue (note your machine may use different network adaptor other than Broadcom as in the following steps. If this is the case, you simply select the driver of your network adaptor in the list)
# Open Device Manager (search Windows Help if you don't know what this is)
# Select 'Network adaptors' and then open (double-click) Broadcom 802.11n Network Adaptor
# Go to the Driver tab and click the Update Driver... button
# Select 'Browse my computer for driver software'
# Select 'Let me pick from a list of device drivers on my computer'
# Select the "Broadcom 802.11n Network Adaptor (Broadcom)" entry from the list, and click Next
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