Hey guys I had posted this question myself now I figured out what the problem was....it was the windows firewall...when I switched on the laptop...I wasn't able to connect to the internet using Wi-Fi...to use the Wi-Fi in xperia u while the laptop is on turn off the firewall till u use the Wi-Fi in ur mobile..or in the list of allowed devices include your xperia u mobile...try this it might work ..if it still doesn't work go to google play n download Wi-Fi analytics app and connect to the Wi-Fi through this app..it connects to the best possible network available judging on the signal strength
...I did this in my xperia u it worked me
please post responses so that I ll know if it worked for others