It happens...usually due to neglecting to maintain your computer. Regardless of what Microsoft says you need to defrag and clean your registry at least once a month. There is software out there to pay for but for those of us that are broke you can use defraggler and ccleaner, which you can get from http://www.piriform.com. Thing is that if you are one of those who thinks your computer will just run forever without any care and now your machine will not even startup due to an error like "windows\system32\config\software corrupt", then you need to be here on my blog asking questions. Take this scenario, for instance, how do you recover from this? Well if you have made a recovery cd/dvd you can fix the damaged file. Most don't do this though and end up calling someone like me to make repairs to their computer. Many newer machines have a partition where you can just reload the operating system, which works too, however in doing so you lose your browser history, saved passwords, software installations etc...pretty much everything except your hard drive data. It means you have to re-install any software you added from the time you bought your machine to that point. Sometime this is necessary to do, but in most cases it can be prevented. Microsoft came out with system restore with Windows XP but that may not always work either if the registry databases are corrupt which me manual restore maybe required. This is all controlled by the registry and four very important files on every Microsoft OS located in the directory <systemdrive>:\<windowsdir>\system32\config\. This directory contains the files that are the heart of your computer, most importantly the system and software registry hive files. If either gets corrupted your computer will not boot regardless. I use ERD Commander in these cases to boot off of a cd/dvd drive external or internal to evaluate the operating system. In many cases if one of those two files are corrupt you will not be able to use restore points or system file repair, which means you have to do it manually. This is how you get booted back up without having to re-install all of your software again, right here in this blog. Steps(will refer to command dir):
1) Boot the machine with either a 3rd party cd/dvd like ERD commander or the original MS Windows cd/dvd where you would choose to goto the recovery console rather than install windows(basically you want to be able to access the harddrive and files on it.)
2) Once you have the machine booted off external media you choose, open the scary DOS or command prompt windows/app. You can use use explorer for this if you use ERD but the Windows recovery console is DOS based.
3) Once you have command prompt eg. c:>\windows type in: cd\ to return to the root of the hard drive files The \ character is always root. Now at "root" you can type cd System Volume Information (at any part of this you can use cd\system volume information as well). By putting a backslash(\) at the end of the cd command you access root, so basically I am telling you that no matter the directory on the system drive (c:>) typing: cd\<whatever directory you want>; the OS will take you there. Keep in mind that this directory is protected in Windows and this process works only if you are booting off original disk or a product like ERD because of this protection.
4) Now in sys volume info directory cd snapshot. Now you are in the area of the hardrive where all the restorepoints are kept. Type dir /p and check the dates on each directory.find the latest directory matching the last time you computer was working fine and cd to it : eg cd <my latest>. Each directory will be named RP# (aka restore point). Find the newest and cd to it.
5) Now in the restore point directory you can copy the registry hives to a disk or to a backup directory you can create on the hard drive of your computer Those files will show here as _restore<etc>. Copy the system and software files(rename them to be system and software) and replace the corrupt files in c:\windows\system32\config.
This is poor descriptions of the issue. I am trying to keep everything in lamans terms and not be too technical. The process is really not as scary as you might think. For me it is a 10 minute process, well really 5, but if you have questions or need help visit me back here and post your questions/comments...I am here to help with any computer questions you have.