On VGN-FS640W screen is very dim, as if it were on battery. Is there some setting I can change to make brighter?

You could be manually brighting it. Hitting the FN and UP arrow and Down Arrows Dim and brightens the screen, But if it does it on it own check your settings on Power Options in the control panel and on the left you will see Create a power plan sure setting are set to Balanced then click on next. You now get to pick to dim it when plugged in or on battery. I change it to "Never" becuase i like to see the screen brighter save your setting and your done.

Most laptops do not have a BIOS button to reset the BIOS settings.
To reset the BIOS just remove the BIOS battery and wait for approx.20 mins for the battery charge to discharge then replace the battery.
OR
After removing the BIOS battery just short out the battery terminals on the motherboard with a paper clip, the replace the BIOS battery and go into the BIOS setting to change the Date and Time etc.

There isnt a fuse for the LCD screen.
If your laptop appears to boot up and the hard disk LEDflashes, then the problem is probably a back light fault. This can be a faultyinverter or a faulty CCFL lamp. Please click on this link and follow myinstructions to identify the fault and possible fix :- http://www.fixya.com/support/r3598095-faulty_laptop_lcd_screen

The main battery needs to be replaced, or the charging ciruit on the motherboard is not working, remove main battery get a stiff toothbrush and clean terminals in the laptop incuding the battery, if no change! check the device manager to see if battery is present, if no! reset bios to factory default.

Sony Vaio PCG and some Vaio VGN Series use the last 7 digits of the laptop serial number to calculate the bios master password. The serial number for a Sony Vaio is often located on the bottom of the laptop on a white sticker. The serial number may also be found by removing the battery and checking in the battery compartment. The link to unlockbiosrecovery.com below works on the following models:
PCG-C, PCG-F, PCG-GR, PCG-GRS, PCG-GRT, PCG-GRV, PCG-GRX, PCG-GRZ, PCG-N, PCG-NV205, PCG-QR, PCG-R, PCG-SR, PCG-SRX, PCG-TR, PCG-V, PCG-VX, PCG-X, PCG-Z, VGN-B, VGN-FS, VGN-S, VGN-T, VGN-TX1, VGN-X
I use http://www.unlockbiosrecovery.com/Unlock_Sony_Bios.php
For $20 they use the last 7 digits of the laptop serial number to calculate the bios master password and then email it to you in an hour. They guarantee it to work and you can call them if you have a question.

This is a Function of the laptop,you need to adjust your Power Settings. See the PC, does that to save Battery power, it will always be brighter, generally, on the Wall supply. You may need to set up a profile for both.

This is quite common actually I see it a lot. There really isn't any reason, it seems that when on batteries it trys to save power by Dimming Down, but it often just mucks up.. I always now set it via the keyboard keys. whenever I change from power adapter, to battery.

I went to the Sony Vaio support site for your model specifically and they mention a battery recall. There is a place on the site requiring specific information about your system and your battery to see if you are eligible for the recall. I would try this route to see if it resolves your issue. If not, I would say it is a blown transistor and you need to replace the motherboard. This means buying a new system, as Sony VAIO laptops are very proprietary and hard to find parts for.

Good Luck,

NOLA Tech

your hardrive is going bad cannot read the disk.take hardrive out and tap it slightly? then replace see if it makes a difference..

Are you using vista or xp?..The laptops to conserve power will dim the screen , you can , however, change it to brighter by pressing the "fn" key which stands for "functions"key. while pressing that, you should have another set of keys either to your right or obove your left finfers with the symbol fo a sun with arrows pointing up and down..press the fn and tap the up arrow key to increase the brightness.

Let me know if that works

there should be an Fn + one of the F keys function to make the screen brighter or dimmer.