Try this and see if the button works from here (outside windows).
1. Turn off the computer.
2. Press the ASSIST button (it 'should' launch the Vaio Recovery Mode). If it doesn't...
2a. Perform a power cycle by poking the "battery off" button at the bottom part of the laptop. Here is a link to an example of where it is..
http/docs.esupport.sony.com/pc/SVP1121_1321_series/EN/contents/TP0000053407.html
2b. Now repeat steps 1 & 2.
If that doesn't work, you may well have start from scratch, reinstalling the system.