There could be hard disk errors that need to be fixed, or there could be files in your OS that are missing. First you should login to an account that has administrator privileges and go to Start, select Run and in the Run window type CMD and then hit enter to bring up your command prompt. If you're using Vista or Windows 7, go to the Start Orb and type CMD into the search bar, then right+click CMD and pick "Run as Administrator." Now, in the command prompt, type SFC /SCANNOW and hit enter. If you're missing any files you will be prompted to insert your Windows disc, it is quite possible that you don't have one, but the main reason you're doing this is to see if you're missing OS files because if you are then that will immediately explain the blue screens and freezes because parts of your Operating System are gone. Anyway, it will prompt you for a specific disc, if you don't have it and you don't know anyone who has it then you can't complete the scan, so stop/cancel it and run it again when you get the CD. For now though, let's work with what we have. We want to see if your harddrive is damaged. In command prompt type CHKDSK /R this will scan your harddrive for errors and repair them on the next reboot (this scan will take at least 40min). After all of that, you should run a PC/Registry optimizer such as Glary Utilities http://www.glaryutilities.com/
(I prefer this one because it's free and simple to use. Tell it to scan after you install it and then tell it to repair the issues it finds.
Scanning with an anti-malware program, an anti-rootkit program, and an anti-spyware program certainly won't hurt anything. The 3 free programs I use are
If you don't have any antivirus program installed, Avast! http://www.avast.com/lp-upgrade-4-5-free
# Is a great one, but it does not work well at all with other anti-virus programs, so don't install it if you have another anti-virus program (such as Norton Antivirus, Panda Anti-virus) running.
Hope this helps!