For general non-boot issues like this here is my process:
Start by making sure you have a back of the data/files on the machine. This usually does a couple of things:
- A.) it makes sure you don't loose anything.
- B.) if you don't already have a backup it gives you piece of mind
- C.) it allows you to just kind of give up and reimage/reload windows.
- D.) if you start trying to backup of a damaged machine and it fails then you know it's probably a harddrive failure of some kind.
Whew, hopefully your data is safe now. Next is to stress test your harddrive. I like to use the Ultimate Boot CD for that because it's free, requires no OS, and is reliable. There are several other tools like it though, including some of the newer live CD's. If your harddrive passes, then you can either keep trying to troubleshoot windows or reload/reimage. If it fails, you have no choice but to replace it and reload.
Assuming you now have your data safe, and a confirmed good harddrive your next step is to see if you can get into safe mode. If you can start looking a selective boot/clean boot, slowly adding things until you can't. Whatever you last added that stopped you from booting is your issue.
If you cannot boot into safemode then personally I think you're best to reload here, why because you're already so unstable you're in trouble. In the XP days you could try a repair install, and in the vista/win7 days they have startup repair options but to be honest I think you're rolling the dice in terms of system stability.
Hope this helps.