Sounds like you've either got some bad sectors on your HDD or your system files are corrupt. Try to boot from your Windows 7 installation DVD, click repair option and then click Command Prompt. There, type 'chkdsk c: /b /x' and after that 'sfc /scannow' (without the ' ').
Edit: Your system drive might not be C:, so when you get to command prompt check if there are Windows and Program Files folders by typing 'dir'. If not, check D:, E: and so on until you find those folders and then replace the c: with the letter you've found the system folders in.