Let's back up a minute. I've recently seen another post with similar issues.
The problem with these errors is that you may not see these bugs until something that interacts with the problem software, SYMEVENT.SYS in this
case, exploits the bug whether it was meant to work that way or not.
Bad Drivers can cause this issue also. I would have you run the memory diagnostic utility from the Vista Bootup DVD
If your memory Tests good, please reboot and Tap the F8 key upon booting the PC, and Arrow down to the Last Known Good Configuration
and see if you're able to boot into Vista. If not, See the following Utility to repair Windows Vista:http://www.bleepingcomputer.com/tutorials/repair-windows-with-windows-startup-repair/
If Neither of those options are effective, Then you'll want to run A hard drive diagnostic utility to check for a bad hard drive.
PS Is this is Desk top or laptop pc?
If a Desktop, is it a custom build?