Did you simply move the HDD into your new system without reinstalling windows?.....I'm guessing you did this.
For many reasons i.e. hardware change (e.g. cpu, motherboard), you must do a full reinstall of windows (I know this will be a pain but you have to do this). Of course, make sure you backup everything you need to first.
If, on the other hand, you did do a full reinstall of windows, did it boot into the desktop immediately after this install?