The vttimer.exe process is used by your Via based graphics hardware. If you are not experiencing any difficulties with your graphics hardware you should leave this process running.
BUT
"It is also a Worm. It came into the wild on 5/31/07.
The virus is a hidden system file, located in %systemroot%\system32.
It creates a services (in services.msc) called "Copyright (C) 2001-2005 S3 Graphics, Inc."
You cannot stop the Service, but you can change it to Disabled. Once you do that, you can reboot.
If you right-click the file ->properties, the summary (Title, author) of the virus/worm will be blank. The summary of the real S3 driver, will have the S3 company information." (http://forums.viaarena.com/messageview.aspx?catid=31&threadid=75881&enterthread=y) More information
