System Restore.

What is System Restore?
System Restore is designed to automatically monitor and record changes made to the core Windows system files and to the registry. System Restore can then allow you to undo (or "roll back") a change that caused instability in your system. This is accomplished by periodically recording a "Restore Point" (or System CheckPoint) that gives you the ability to roll your system back to the point in time when your computer was known to function properly.

System Restore is not intended to be an "uninstaller" or a backup program. If Windows does not function properly after installing software or drivers, you should use the Add/Remove Programs tool in Control Panel (or use the program's uninstaller) to remove the software before using System Restore.

File Types that stored at the time of system restore

System Restore monitors most system files with .exe, .vxd, .dll, .com, and .sys extensions. It does not monitor user-created files (for example, files that have .txt, .doc, or .xls extensions), the My Documents folder, Temporary Internet files (including the Internet Explorer History, Cookies, or Favorites files), the Recycle Bin, or the Windows Swap (.swp) file.

disk space
The size of the Data Store is determined by the size of the Windows drive and is as follows:

•	The minimum size of the Data Store is 200 megabytes (MB). If your Windows drive has fewer than 200 MB of free space after you install Windows Millennium, System Restore is disabled by default and you will have to manually enable System Restore after you have freed up sufficient disk space. If System Restore is disabled due to the low disk notifier, System Restore automatically re-enables itself after sufficient disk space is made available.
•	On logical drives that are less than 4 gigabytes (GB), the maximum size of the Data Store is 400 MB.
•	On logical drives that are 4 GB or larger, the Data Store is approximately 12 percent of the logical drive that Windows Millennium is installed on.

Note: Once the Data Store reaches 90 percent of the allotted size, it begins to "purge" older Restore Points on a first-in first-out (FIFO) basis until the Data Store reaches 50 percent of its allotted size. The actual number of Restore Points available after this purge occurs may vary, based on what software was recently installed.

To manually adjust the size of the Data Store, follow these steps:

1.	In Control Panel, click System.
2.	Click the Performance tab.
3.	Click the File System button.
4.	On the Hard Disk tab, adjust the System Restore disk space use slider to the preferred size

Registery

•	HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Sr This key is related to the System Restore filter. This key and its values must not be be modified under any circumstances as any modifications can cause the operating system to malfunction.
•	HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Srservice This key is related to the System Restore service. This key and its values must not be be modified under any circumstances as any modifications can cause the operating system to malfunction.
•	HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\SystemRestore In this registry key, you can modify some DWORD values. However, in this key, there are some values that must not be modified under any circumstances.