|The contact pad or whatever under the key is almost worn out. |
Laptop keyboards are well known to malfunction sooner than desktop keyboards.They are a frequent replacement item, and relatively inexpensive.
Just search the web for: VGN FE90 replacement keyboard
Note that the Sony part number of the keyboard varies depending on the keyboard layout / language and the color of the keys - remove your keyboard first to find the correct part number on it, before you search, or you might be able to find that out on the Sony web site by submitting or using a specific model code or serial number for your model .
If you don't mind replacing the keyboard yourself......
There are free or cheap Sony Service manuals for your model or model series available on the web, if you need info about how to remove or replace the keyboard, or sometimes you can just search for something such as: how to replace Sony VGN FE90 keyboard and you'll find something.
In the mean time, plug in a USB keyboard, or if your laptop has a PS/2 port, a PS/2 keyboard - it will work fine.