As Windows was preinstalled, you should have a Repair option in the Advanced Boot options. Restart the computer and press and hold the F8 key as your computer restarts. Look for Advanced Boot Options or similar wording then go to Repair your computer. You need to run a Startup Repair and/or chkdsk from the command prompt option.
Optionally, download and create a Recovery Disk (32-bit x86 or 64-bit x64) here click here After downloading the ISO image to your hard disk, burn it to a CD (a DVD is not necessary as the 32-bit version is 120MB and the 64-bit version is 148MB). On the CD, there are two folders (Boot and Sources) and one file Bootmgr.
It cannot install Vista but it repairs an installation in the same way as the Vista DVD, i.e. boot from the disk, choose ‘Press any key to boot from CD/DVD’ and Vista will start to load the Windows files. Select your language, time, currency and keyboard then click ‘Repair’ in the bottom left corner. There are five choices within Repair: Startup Repair, System Restore, Backup Restore, Memory Test and Command Prompt.