From what I understand from your later posts, it seems the tech transplanted your old drive into a box with new processor and motherboard. (post 14 above)
If so, then your tech is not too clued up. With such a major change, you always install Windows from scratch, as the old installation is tied pretty tight with the old system, more specifically the motherboard, chipset and hard drive controller.
Although it has been done successfully to just transplant Windows, there is no guarantee how stable it will be over long term use.
If it isn't a tansplant hard drive, it may be faulty dvd-rom drive or faulty memory. Both can cause such issues.