Hi
I have also faced same kind of problem after upgrading my Iphone 3gs to ios6.1.3 ,
earlier wifi connection which i was using on my Iphone 3gs, was using Security Type WPA2, which was shared from my laptop via "Microsoft virtual WiFi Miniport Adapter", was working perfectly till ios6.1.2, but when i have upgraded my iphone 3gs to ios6.1.3, my iphone 3gs became unable to join wifi,
Today I have created "adhoc network" on my laptop using "Broadcom 4313 802.11b/g/n"(default wifi adapter of my laptop), and kept Security Type "WEP", and found my wifi working fine with my iphone 3gs,
So if someone using LAN internet on their laptop, they can share their LAN internet to use on their ios devices, via creating an 'adhoc network',
How to create an 'Adhoc Network',
choose Security Type : WEP (compatible with ios6.1.3)
http://www.youneedabudget.com/support/article/create-an-ad-hoc-network-in-window s