you may need to do repair install of Vista. it will restore all the system files and leave your data and settings intact. You will need windows Vista installation disk put it in disk drive and boot from it.Boot from CD - How to Boot your Computer from a Bootable CD
once you know how to boot from bootable installation disk. put the disk in drive and restart the computer. follow the steps given in this article for repair install
Windows Vista Repair
it will resolve the issue
if you donot have Vista disks
it will resolve the issue. Ask me if need more info.
Computer Support Specialist
MCA. M.Phil. in computer science with tons of experience of computers.