Hello,
1).Run last known good configuration by hitting f8 key while ur system boots.
2)Do system Restore.....from safe mode...start>programs>accessories>sytemtools>systemrestore.
3).after doing this also if u can't succeed ....then from other computer download MWTI Tool kit by typing in google search....this mwti tool kit can be downloaded from
www.mwti.net. run this .exe from safe mode ur problem will be solved in next boot.
4).if above three options didn't work then simply format ur system.by taking backup from safe mode.
5).if safe mode also not working then.....find a linux OS based cd boot from that and backup ur system...and then format.
6).if u can't find any linux based OS cd ....then simply remove ur hard disk and connect to other system..then take backup and install OS after format.
Regards,