Hello,
Since when this is occouring?
Also, do you have the installation/recovery disk for Vista?
It might be a case of file corruption. First of all we will try a system restore, as you told you can start the system in safe mode. Start the system in safe mode and do a system restore to a date prior to this problem occoured.
Go to this link to get a guide on how to do system restore: http://tinyurl.com/2qhd77
If that doesn't helps, reply here for further help.
Computer Systems Analyst
Professional computer engineer & working as systems developer with top multinational company