can you not boot up and log in? if you can get into windows and log in run this Perform a SFC Scan (System File Check)
1. Click Start, click All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow
2. Type the following command, sfc /scannow
and then press ENTER:
(A message will appear stating that 'The system scan will begin'. Be patient because the scan may take some time)
3. If any files require a replace SFC will replace them. You may be asked to insert your Windows 7 DVD for this process to continue
4. If everything is okay you should, after the scan, see the following message "Windows resource protection did not find any integrity violations"
5. After the scan has completed, close the command prompt window, restart the computer. (TIP repeat scan 3 times) How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7 >>> How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7 Parameters
/scannow : Scans all protected system files immediately.
/scanonce : Scans all protected system files once.
/scanboot : Scans all protected system files every time the computer is restarted.
/revert : Returns the scan to its default operation.
/purgecache : Purges the Windows File Protection file cache and scans all protected system files immediately.
/cachesize=x : Sets the size, in MB, of the Windows File Protection file cache.
/? : Displays help at the command prompt.