Why does my screen flicker only in Windows 7?

To me it looks like hardware failing, either the graphics card or monitor electronics. In laptop those two obviously are tied. The only thing you can do to it yourself is to check that no part of the system overheats; but most likely that only postpones the inevitable, ie. replacing the hardware is in order sooner or later.

For reasons why it only happens in Windows.. start by checking the refresh rates; if Windows tries to use higher refresh rate, it might make a soon-to-be-failing-totally hardware beyond the point of failure. Temperatures of course can also differ between OSes, because you run different amounts of eye candy. Also the exact pattern of loading the hardware varies due to different drivers etc.