Blocking FPS on Vista

Hello,
A few days now since my fps (frame per second) in games are blocked at a value between 30 ~ 36 fps whatever adjustments videos games are played.

I am under Vista with the following configuration:
Intel Core 2 Duo E8400
Geforce 9800GTX
4 GB DDR2

Everything was going well so far, I measure my FPS with the software FRAP and I exceeded almost all the time on the 100 FPS games that I played. And now it no longer exceeds the 30 or so.

I've uninstalled and reinstalled my video driver, but it changes nothing.

I checked the active processes running in the background, nothing abnormal.

Someone would have an idea of what has happened to my computer?

Thank you in advance for your help

Is this happening on the same game or some newly installed games?
You recently upgraded the driver for the video card?

yes it is the same games (Oblivion, Half Life 2, Medieval Total War 2)

And no, I have not changed my graphics driver.

The vertical sync may be activated? in this case, it blocks your FPS which has a fixed value ...