During bootup your OS will load any drivers and system files necessary for the correct operation of the system. If a driver or files are either corrupt, not necessary or the wrong ones, the screen will seem to freeze while it tries to figure out what to do. Once it's obvious the drivers or files are no good it will eventually continue. If you recently changed devices or added some software, that could be the problem. First though try repeatedly hitting the F8 key while booting to get to the safe mode screen and select "last known good configuration". that has got me out of trouble several times. If that doesn't do it, go into the control panel and open the Administrative tools and then look at the Event Viewer for a clue to any errors that are ocurring that may help you to figure out what's happening. Since during boot the system is not active yet, I wouldn't expect to see the error there, but perhaps in Applications. If there is an application responsible it may error when it tries to run after boot.
You can also try going into msconfig > startup from the run box and try to determine what item if any is causing the problem during startup. You can usually turn off and back on most things in there without issues, at least for boot. Audio apps and graphics tools will not come on if you turn them off but will not effect your immediate system (maybe raid if you use it but not too much can go wrong otherwise).
i did all.safe mode- last known good configuratiuon ; administrative tools and also unchecked applications in start up which werent necessary but to no avail.this is very sad... thanks but.