Have not been able to log in

I have not been able to log in for a month now, every time i try and log in it either crashes at the login screen before i even click the log in button, or after i do gets to initalizing multimedia and then poof crash, i have tried clearing cache, unistalling, turning off firewall and virus programs. Have never had any problems logging in, in the 3 years i have had it on my computer till last month. Any help would be great. I have had the same computer for about a year and a half and untill the problem almost 2 months ago now it ran fine, but still cannot log in on any veiwer.