Event Viewer is missing events

This could be caused by system or hard drive corruption (or perhaps malware).  Let's check for both:

Go to Start / All Programs / Accessories / Command prompt and right click on command prompt and click run as Administrator.

Type sfc /scannow and enter and let it run.  It will scan and try to fix some of your system files.  Hopefully it will complete with no corruption it could not repair (if there is such corruption post back here or try to analyze it to find the problem file(s) using http://support.microsoft.com/kb/928228.  Try to post any corrupted files here so we can see if they can be repaired with good copies from the installation disk (unless there are too many).

While in Command Prompt, type chkdsk /f /r and enter and let it run.  It will want to schedule itself to run at the next restart.   Answer yes and then reboot to run the program.  It will scan and try to fix any corruption or bad sectors on your hard drive and mostly remove that as a potential cause.

You may be infected with malware. Try to run anti-malware programs (in safe mode with networking if necessary). To fix this problem (if it is a problem) download, install, and run the following two programs:http://www.malwarebytes.org/mbam.php andhttp://www.safer-networking.org/en/index.html.  You may also want to try a free trial of Pandasoft (http://www.pandasecurity.com/usa/homeusers/solutions/global-protection/).  Before downloading pandasoft, uninstall whatever anti-virus program you are currently using (because installing and running two at the same time can cause conflicts, freezes, and all sorts of problems).  Once you've uninstalled your current AV program, download, install, update, and scan using Pandasoft.  I think you might be amazed at how much it finds that the other software missed.  Once complete, uninstall pandasoft and re-install your current AV program (unless you decide to switch which I did when I first tried it about 4 years ago).  You may also want to try the new, free Microsoft Security Essentialshttp://www.softpedia.com/progDownload/Microsoft-Security-Essentials-Download-131683.html (with the same caveat that only ONE AV program can be installed and running on your system at any one time).  You may also want to try the free Avira at:http://www.free-av.com/ and Avast at: http://www.avast.com/index.  Reboot after completing all the scans.  You may also want to try the free OneCare athttp://onecare.live.com/site/en-us/default.htm and let it run all the options (except the registry cleaner) because that’s good maintenance (it will take some time to complete but can be done in the background).

If that doesn't work (or if SFC found corruption it could not repair), we'll need to do a system repair/upgrade using the genuine Windows Vista Installation Disk (one you own or one you can borrow from ANYONE).  Here's the procedure:http://www.vistax64.com/tutorials/88236-repair-install-vista.html.  Although this will not affect your data, settings, or programs, you should still backup your data before starting just to be on the safe side. You may have a lot of updates to re-install (including any service packs you had to remove).  If the version on the system came with SP1 or SP2 pre-installed and the disk is an earlier version, then you'll need to make a slipstream disk as follows:http://www.vistax64.com/tutorials/151606-vista-sp1-slipstream-installation-dvd.html.  

Hopefully one of these procedures will resolve or at least identify the cause of your problem.  If not, post back with results from what you tried and copies of some of the scans and we'll try something else.  Post back in any case so we'll know how it turned out.

Good luck!