Ok... Here's one thing that many people don't know:
When wiping your system to reinstall, you should always completely delete the partition your operating system was installed on, as if you had some nasty stuff on there, it may not be eliminated by just formatting.
In fact, to truly make sure your drive is clean, I recommend you use a utility called 'Darik's Boot And Nuke (DBAN)' that safely and completely obliterates the ability to read any of the previous content. It was first recommended to me by a pair of guys that do a video blog for (if I recall) PC Magazine.
It can be downloaded for free here, but you will need to make sure you get the version that is meant for the type of media you are going to use to deliver it to the intended drive. (Note: Make SURE you have gotten all desired information off the drive, as there is NO KNOWN WAY to recover data that has been wiped in this way. The program changes the start and read locations the drive uses to access information, and thus CANNOT ever return to the original start point once it is done)
Next, don't use Repair install... Just format the partition you want to use for the OS (after you have created it in the setup section of the Windows install program)... Oh yeah, if the PC has more than one drive installed (including the DVD drive) you should stop the install program (by shutting down and restarting) after you have created and formatted the partition to make sure the system calls it 'C:', as while installing to a different letter will usually work, many programs will not function properly if the operating system is not installed in the drive called 'C:'...
Now if you go through all this, ensuring a clean install, and you know your OS disk is good (not all scratched up), as is your CD Key for the OS, and you still encounter the same problem, it may very well be your memory, or the drive itself may be bad... Or heck, it could even be the powersupply (really, the symptoms for an overloaded or failing power supply can appear to be any number of hardware or software problems)...
I hope you solve your problem, and I hope I've given you some areas you can check, and that everything works out for you.