It can be dangerous but during your computer life it will almost certainly be necessary to dabble. In fact, you probably already have without realising it! Whatever you do, there is a golden rule - BACK UP THE WHOLE REGISTRY FIRST!!! There are a few built in safeguards within Windows but most of these depend upon either being able to boot to Windows or being able to use the Recovery Console to effect a repair. The one safeguard that doesn't entail either of these is to bring up the startup menu at boot by hitting F5 or F8 just before Windows starts and choosing the "Last Known Good Configuration" option.

An indispensable tool if you want to experiment with the system is Acronis True Image or Norton Ghost.  Acronis TI 7 was given away free some time ago and it may still be around on magazine cover disks. I berlieve I have a few copies lying around somewhere. If you plump for Ghost, I'd look for an older version - 8 or 9 are OK.

What these programs do is create an image of your drive/partition and store it in a location of your choice. If something untoward happens, you simply boot from the created CD or floppy, wait 10 minutes or so and your system is back in all its glory. The big manufacturers use similar solutions for their "restore disks".

To take full advantage  of these programs, it's wise to partition your hard drive into a "boot" partition which contains only the operating system and programs and then as many other partitions as suits your needs. These other partitions are used to store your personal data such as emails, favourites, documents, address book, music and photos etc.

Once this regime is in place and an image is taken, you can play to your hearts content.