Hi colchicus,
Method 1:
You may try to run a SFC scan to check and fix for any corrupted system files and try installing SP1.
You may refer the following KB article to run the SFC scan:
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
http://support.microsoft.com/kb/929833
Method 2:
You may also try to run a check disk to repair problems related to bad sectors.
Note: When you run the check disk, there are chances of losing data so you can take a backup before running check disk.
For more information, you may refer the following link:
Check a drive for errors
http://windows.microsoft.com/en-us/windows7/Check-a-drive-for-errors
Also refer the below links:
Learn how to install Windows 7 Service Pack 1 (SP1)
http://windows.microsoft.com/installwindows7sp1
Steps to follow before you install Windows 7 Service Pack 1 from the Microsoft Download Center
http://support.microsoft.com/kb/2505743
Hope this information is helpful.
Praseetha K
Microsoft Answers Support Engineer