If something is hard-locking your PC, you likely have one of three issues:
1. Bad drivers - These can be a source of a multitude of problems, especially if either the uninstallers do not clean things well or the installers leave components of the old drivers in place such that Windows will use them along side the components of the new drivers. Having had driver issues like these in the past, I know two possibly solutions. The first and simplest solution is probably to do a completely reinstallation of Windows by wiping everything clean on the hard drive and installing the latest drivers from the start. The second and more complicated, but possibly easier solution is to remove all traces of your existing drivers from your machine and then install the latest drivers. This all assumes that the latest drivers from your hardware manufacterer fixes your problem.
2. Bad hardware - This is a problem that is a pain to track down, but can cause alot of issues. Bad graphics hardware will cause system lockups when stressed, as it does not interact with the driver proper (hence why it is bad). Bad add-in cards can prevent programs from closing as the kernel mode components programs use (in their interaction with the drivers) can freeze, preventing Windows from being able to terminate the program, even if the user-mode portion is terminated. It is difficult to track the cause of the problem, but in your case, I assume, if the drivers are not the cause, it is the cause of something between your northbridge chip and your graphics chip, if it is not a problem in your graphics chip. One possibility is that you have an overheating problem which is worth exploring, but if you have overheating hardware, it could very well be damaged and therefore need to be replaced.
3. Bad operating system - Operating systems that are bad can be a source of numerous difficulties. If you run Windows Millenium Edition or Windows Vista, or even an older 9x operating system like Windows 95, and the other two possibilities are not the cause, the problem is likely your operating system. This can be resolved by reformating your computer and installing Windows XP. Even if the operating system is not the cause of the problem, if you use one of the operating systems I mentioned, I highly recommend reformatting your PC and installing either Windows XP or Windows 2000. A properly configured system that has one of these two operating systems will likely avoid many headaches the operating systems I mentioned above are prone to developing.
I know people are probably going to jump on me for the Vista comment, but since Vista has been a source of headaches for so many people, it is hard not to poke fun at it at times, kind of like the Windows RG Edition satire of Clippy from Microsoft Word.