If you have the product ID, you should have no real big trouble... To get the product ID, download Belarc Advisor or other computer inventory. Then use an OEM disk (you can even borrow one, if you have that product ID.) Then when you activate, activate from the same telephone number, if possible.
Usually nothing happens except activation. But you may get an invitation to call Microsoft to clarify what you are doing. They are nice about it. But if you do it again, your argument must be better.
You have the right to use the same product ID, if you upgrade your hard drive, or for other system failures. It is when you activate from different computers and telephone numbers and area codes that the problems arise. The activation code tells the Microsoft folks a lot of details about where and what was used for the installation, as well.
IF/When you get the choice to activate online or by telephone, call them.