Like Chromebuster said, your computer may just be giving you a hard time, or this may be caused by the trojan horse that your computer is infected with.
Assuming that this problem is being caused by the trojan horse, I'd suggest that you run both an antivirus scan and an antispyware or antimalware scan, in order to try and remove it so that things can be back to normal again.
The antispyware program that Chromebuster suggested, SuperAntispyware, can be downloaded from the link below for free and may be able to remove this trojan horse.
If you need a free antivirus program, I would recommend that you try using Microsoft Security Essentials, which was created by Microsoft, and can be downloaded from the link below.
Just so you know, I wouldn't suggest that you have more than one antivirus program installed on your computer at the same time, as they may end up conflicting with each other if you do, so if you're going to install Microsoft Security Essentials, I would suggest that you uninstall any other antivirus programs that may already be installed on your computer.
It is alright, however, to have both an antivirus program and antispyware program installed on your computer at the same time and they shouldn't conflict or cause any other problems.
Also, perhaps you should consider downloading and using either Mozilla FireFox or Google Chrome, both of which are free browsers that are safer than Internet Explorer and can also be customized with a variety of free add-ons and themes.
Good luck and I hope I helped you!