Developed by Kaspersky Lab, TDSSKiller is a FREE handy tool that can quickly detect and remove both known and unknown rootkits, which are programs that can hide the presence of malware in your system.
Installing TDSSKiller is easy and will scan your system in only about 15 seconds. To get your free copy, simply enter your email address in the form to the right and hit the submit button. Instructions on how to download will be emailed to you.
A rootkit for Windows systems is a program that penetrates into the system and intercepts the system functions (Windows API). It can effectively hide its presence by intercepting and modifying low-level API functions. Moreover it can hide the presence of particular processes, folders, files and registry keys. Some rootkits install its own drivers and services in the system (they also remain 'invisible').