If your computer crashes it will create a dump file. From this dump file you can diagnose the source of the problem. It is unlikely you will need this dump file; if you do, you can always turn the option back on. This guide will show you how to save space by disabling memory dump files in Vista.
Disabling Dump Files
1. Press Start and click Control Panel
2. Type Advanced in the search box
3. Click on View advanced system settings in the search results
4. Under Startup and Recovery, click Settings
5. In the Write debugging information drop down, click (none) and press OK
You will now save space and not store useless information.