Well no idea what's caused that, you could try getting the bootmenu holding f8 and seeing if you can get into safemode, if you can do that then its just a matter of getting hold of some tools that can fix possible causes.. ie run checks for virus/trojan etc.. and depending on what is found from that, do fixes.
Really helps having another computer with internet to get help etc. Personally if you've got some other pc to backup your hard drive, then I'd do that asap, then trash your OS and format the drive.. reinstall OS fresh... best way really. Fixing your own pc is great way to learn techy shit fast.