Try Safe Mode First
When your PC just won't boot into Windows at all, you'll probably want to start off with Safe Mode—use the F8 key right when the system starts up to access the menu and see if Windows will boot into Safe Mode. If you're able to get into Windows, the problem is likely something that would load only when you boot normally—graphics card drivers, junk applications, or perhaps some spyware applications.
Even Safe Mode Doesn't Work
If you can't even get into Safe Mode, you should use the Startup Repair tools, which can be accessed from the Advanced options in the Safe Mode menu; you can pull out your system repair disc, or your Windows installation disc and use the Repair option. If you're having a bootloader problem that says "bootmgr is missing", you can use the command prompt from an installation CD and use the bootrec /fixboot command.
Use System Restore
System Restore in Windows XP might not have been the greatest tool, but since Windows Vista came around, it's been an excellent choice for fixing up all sorts of problems, including issues that cause your system to not boot anymore. While testing, I once deleted a bunch of files in system32 to purposely cause a blue screen of death, and then used System Restore to get the system up and running again in minutes. If you're unfamiliar with the process, you can follow this step-by-step guide to restoring your PC with System Restore.
Try Booting Off an Ubuntu CD
Whenever I'm having issues with a PC not booting up properly, one of the first things I do is grab an Ubuntu Live CD and try and boot from that, and see how well the system is working. If everything is working in Linux, then you know the problem is likely just your Windows installation, and not hardware related.
PC Crashing or Blue Screening
When your PC is crashing with the blue screen of death at random intervals, you should make sure that you first disable the automatic reboot after the blue screen, and then write down the error message so you can Google it later. If you're able to get back into Windows, you can try using thepreviously mentioned BlueScreenView tool to give you some extra information about the error message, but your best bet is usually to do a search.
If you're using Windows 7 or later and you're getting a blue screen of death, chances are very high that the problem is either driver or hardware related—contrary to legend, Windows just doesn't crash like that very much unless the problem is tied to hardware somehow.
Once you've identified the driver causing the problem, try and head into Safe Mode and uninstall or upgrade the driver causing the problem—sometimes the driver won't be visible, so you'll need to use a command-line trick to show hidden devices in Device Manager. If you're doing a bunch of work in Safe Mode, sometimes it's helpful to force Windows to load into Safe Mode without using the F8 key every time.
No Luck? Recover Files and Reinstall
If you simply can't get anything else to work and you don't have a full backup, your best bet is to pull out an Ubuntu Live CD and back up files from your PC, and then start the process of reinstalling Windows again. If you're using the native Windows backup tools, you can restore your system from an image backup, or use the standard backup and restore process.