I have a HUGE problem with my computer!!!!?

A few days ago, I atempted to update my old computer with the newest software. But now, strange is happening. When I open my brower, ads keep reappearing everywhere from sites I don't know heard of, even on my FB page! I tried everything, like deleting programs I don't use, and System Restore. To make things worse, 30 minutes after I log into my account to my computer,my computer crashes,and I see a blue screen that has a code xc000000021. Please help me! I cant afford to re-program everything in my computer again! I have a Dell Windows XP Home Edition SP3 and my browser is Firefox