Why do you care? Well, SafeMode can be a great troubleshooting tool. If you add a new device or program and suddenly you can't get Windows to boot properly, you need some way to get into Windows long enough to at least undo the damage. By booting into SafeMode you can usually bypass the driver or service that is causing the problem and remove or uninstall whatever is causing the problem so you can get back into Windows the normal way.
SafeMode is also a valuable tool when it comes to eradicating malware- particularly worms- from your computer system. Often, even if you update your antivirus software to detect the latest threat it can't do anything to remove worms that are running in memory. Rebooting your computer won't help because the worm will generally already have loaded itself in memory before you can do anything about it. If you boot into SafeMode though the file that starts the worm will not be allowed to start and you can then run your antivirus software scan to detect and remove the malicious files.
So, how do you get to this magical SafeMode? Well, the "standard" way is to press the F8 key on your keyboard when Windows starts loading. This can be quite tricky on many computers where you may have a window of opportunity of approximately .2 milliseconds in which to press the F8 key. If you miss the window of opportunity you need to reboot and try again.
If you can't even get into Windows you have little choice but to continue trying to press F8 at just the right moment to enable booting into SafeMode. However, if you can at least get into Windows, you can use the MSCONGIF utility to help you out.
Click on Start, then Run. In the Run dialog box type "msconfig" and press enter to start the MSCONFIG utility. MSCONFIG offers a variety of options and settings you can use to selectively enable or disable certain programs or services to try and troubleshoot and narrow down problems you might be having. You can view the contents of the System.ini, Win.ini and Boot.ini files and check or uncheck the items you want to start. You can also uncheck services or programs that startup when Windows loads if you don't want them to start.
For our purposes, we just want to use MSCONFIG to boot into SafeMode so we can run our antivirus software scan or a virus / worm removal tool to get rid of malware on the system. If you click on the Boot.ini tab you will see some checkboxes at the bottom under Boot Options. Click the checkbox next to /SAFEBOOT and select Minimal (if you really need network access while you are in SafeMode you can select Network instead).
After you make the changes the computer will need to be rebooted for the changes to take effect. The computer will boot into SafeMode and allow you to do what you need to do to clean any malware or other malicious software from your system. Once you have completed the actions or tasks you needed to use SafeMode for, you need to reopen MSCONFIG following the instructions above and uncheck the /SAFEMODE box under Boot Options so that the computer will boot into normal Windows mode the next time you start it up.
If you have some extra time on your hands or want to use MSCONFIG for other troubleshooting or maintenance you might want to understand what all the rest of those options and checkboxes are for. You can refer to this Microsoft technical support article How to Troubleshoot By Using the Msconfig Utility in Windows XP.