Can't Log in! Help Please!!


Hi all, I have a Macbook dual processor that originally came with Tiger, but now is running Snow Leopard. I think I may have a virus because after downloading a file off the internet, I can't log in anymore. Every time I type my password in (correctly) a blue screen pops up, and it immediately kicks me back to the login screen. It does the same thing when I try to log in in safe mode. If possible I'd like to keep all of my files and info, but if needed I can wipe it out. I no longer have the Snow Leopard CD, so if I do need to wipe my computer, I'd like to do everything possible to do it without deleting Snow Leopard and restoring it back to Tiger. I DO have the Snow Leopard start up disk for a Macbook Pro if that makes a difference. How can I fix this?? Please help!