Before buying new hardware check its compatibility with your system. You might be able to determine this at Microsoft's Hardware Compatibility List . My 2 cents Leave a new computer on all night. If it's going to overheat, let it happen while under warranty and before you have invested any time and effort into it. I have seen a recommendation to leave a new computer on for 72 hours while new and fully returnable.
Heat is a big cause of problems with computers.
Test the hard disk thoroughly. Every hard disk vendor has their own utility for diagnosing the status of their drives. Get this utility and run it. You could check the system documentation or Windows device manager. IBM has a free utility called EZ-Smart that does a little hard disk diagnosing and also displays internal information about a disk drive such as the manufacturer.