Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
Tro.Activate_crack is a trojan downloader used to download and install a number of malware threats. This trojan installs itself through an active X control from crack sites. Troj.Activate_crack takes control of your desktop and changes the background. Troj.Activate_crack then displays ads all over your computer making exploring the internet very difficult. What makes this trojan different from other trojans like it (such as Winfixer, Puper, Vundo, etc...) is that the desktop is altered by an html page, not a hidden window.
You'll notice that you are unable to change your desktop from the control panel when you have this trojan. There will be a desktop.html file in the background listbox when trying to change the background. Search your computer for desktop.htm and delete it to get that image off your computer.