Windows Vista includes a utility that will scan your system for corrupt, changed or missing system files. Running this from the command prompt is much easier than booting off the dvd into repair mode.
To run this utility, you will need to open a command prompt in administrator mode. You can do that by right clicking the Command Prompt in the list.
Once you have an administrator command prompt open, you can run the utility by using the following syntax:
SFC [/SCANNOW] [/VERIFYONLY] [/SCANFILE=
The most useful command is just to scan immediately, which will scan and attempt to repair any files that are changed or corrupted. You can run that command with this command:
Alternatively, you can just choose to verify a particular file, or you can do a verify only, which will check but not repair anything. Here's an example:
Note that this utility exists for XP as well, but has different options.