Devil May Cry 4

I get to Fortuna Castle on Mission 3, but when I get to the doors above the area where you fight the giant frog(its a path where you fight 2 frosts) and open the door the game freezes. The door opening sound effect still plays, and other sound effects and music continue to play. Then at another place in the same mission, I'm able to enter a room( the room with the fire balls and table covered with breakable objects) but I cannot exit the room from the way I came. The sound effects and music continue to play here as well. I've lowered the settings, upgraded my RAM, the game runs fine up until this part(and whenever their is too much backgroud visuals, where it will slow down). Here are my computer specifications:
Dell Inspiron 1525
4 GB of RAM
intel Celeron Processor
Mobile Intel 965 Express Chipset Family
If you need anymore info just leave a comment... PLEASE HELP ME!