A family of Microsoft relational database management systems designed for ease of use.
It sounds like your hardware changes were large enough to cause the activation code to trigger. It is possible that the re-activation is blocked by the same problem that is causing the instability. Your best best at this point would be to contact customer support directly, as installation issues are no charge to you, and working with a technician on the phone is a more appropriate way to attack the problem and get your system fixed. You can call 1-800-Microsoft and request support assistance to get routed properly.
You may find that fixing the instability still leaves the product un-activated. In that case, try requesting the program to activate "By Telephone" instead of over the internet. You will dial into a voice response system, and if it cannot activate you immediately, you can then talk with an activation person, explain that your hardware failed and had to be replaced, and they will be able to provide you with a code to get going again.
Good luck!