Why?

If your computer is crashing and you don't know why, restoring a recent working registry can be very helpful and will often fix your problems.  Be aware though that this will most likely cause software that was installed after you backed up your registry to not work properly.  Of course, it won't remove any files or anything, but the registry is like Windows' brain. You took out its memory of when that program was installed, so as far as the registry knows, that software was never installed (even though its files exist on the hard drive).  You will just have to reinstall the software after restoring your registry. This tutorial assumes you have backed up your registry according to the guidelines in HowTo Backup Windows Registry.

How?

1. You can do this similarly to how you backed up your registry.  Again, there are two ways (actually many) - a simple way, and an effective way. First, the easy way:

2. Go to My Computer-C:-backup. Click and drag to highlight all the files there that you want to restore.  Right click on them and go to copy. Now, close that window, go to My Computer-C:-Windows. Then right click and go to paste.  Say yes, you want to overwrite. Now for the safer way:

3. Boot your computer with a boot disk or by pressing F8 just before your computer says "Starting Windows..."  Choose Command Prompt Only as your boot option. Now type in these commands in sequence to restore your registry:

cd c:\windows     (changes directory to Windows)
attrib user.dat -h -r   (makes file not hidden/restricted)
attrib system.dat -h -r     (same)
cd c:\backup     (changes directory to your backup dir)
copy user.dat c:\windows     (copy file to backup folder)
copy system.dat c:\windows     (same)
copy win.ini c:\windows     (same)
copy system.ini c:\windows     (same)
cd c:\windows     (changes directory to Windows)
attrib user.dat +h +r     (gives attributes back to file)
attrib system.dat +h +r     (same)

Now you have properly restored your windows registry. Power on your computer and see if your problem(s) are fixed. 
*TIP: You can copy and paste the commands above into a file in Notepad with slight modifications (no "cd" commands. Instead just use full paths for each command i.e. copy c:\backup\user.dat c:\windows). Then save the file somewhere as restore.bat. Then you can just run restore.bat anytime to restore your registry (preferably after booting to DOS).