Did you make a "System repair disc" with the built-in "Backup & Restore" utility in Control Panel when your PC was working okay? For your sake I hope you did, it's a useful tool for OS rescue under some circumstances.
If yes, boot your PC from it, choose the "Repair" option, then click "Next".
In the System Recovery Options dialog box, click "Command Prompt".
At the command prompt, type bootrec /rebuildbcd and press <Enter>
The Boot Configuration Data (BCD) will be rebuilt.