SO, the problem is my bro installed the new Yahoo messenger, and after he logged off and I logged in to my account (which is the admin. account) programs are not working properly. eg. my utorrent is reset. Nokia pc suite gives error. Msn messenger doesn't log in. Firefox will not open(I'm currently running it as the admin), even the library tab in the taskbar is not working. The same thing happened few days back (same reason, Yahoo messenger installation) but then I restored the system to an earlier point, but right now I forgot to create an earlier restore point. And nothing is working.
Also, when I restarted the computer, after I uninstalled Yahoo, when I logged in to my user, the windows gave an error that 'system.ini is not working or something like that.'
P.S. I just checked, none of the programs are working. Like right now I opened itunes, and it gave the error that I do not have the privilage to access it.