When your computer gets a virus, that virus tries to spread, and eventually it will damage the host making it much easier to detect. A rootkit on the other hand is designed to hide certain elements such as files, processes, registry entries, or network connections, from the user and other programs thus making it very difficult to detect. This technology can be used for good as well as malicious purposes so it  is important to be familiar with your computer to avoid deleting these legitimate objects. Within Windows rootkits are used to hide malware so that their execution goes unnoticed by your security applications. So imagine that a rootkit has been installed on your computer and that its purpose is to hide a virus, thus giving the malware time to complete its goal, steal your data, and damage your system all the while going undetected. Unfortunately, rootkits are extremely effective at this, which means that even though you may believe your PC to be totally clean, some of you could be infected right now.

Most of the anti-virus vendors have integrated anti-rootkit technology into their more recent products. However, this is not a fool proof solution against rootkits because just as the AV companies improve their products detection abilities, so the malware creators find new ways to avoid detection. So as security conscious users we must rely on third party tools to help us, and there are several free applications which specialize in the detection and removal of rootkits. Keep in mind that none of these products will detect every single problem, so it is always a good idea to keep more than one of them to hand.
There are a lot of anti-rootkit programs available, but unfortunately not many of them will work on Windows 7 yet. A lot of this software is very advanced and requires an experienced and technical minded user who is familiar with computers and operating systems. However, there are a couple of options that do not require much technical ability and are also very effective.
At the top of the list is Sophos Anti-Rootkit. The program has a small but easy to use interface with no options other than choosing where you want to scan. As it scans it opens up to a slightly larger interface where it lists the results of the scan and gives you information about each result as well as a recommendation for them. Additionally, a small help file is available that explains the program in a little more detail and gives directions on how to use the command line anti-rootkit tool which is also included. For all Windows users, Sophos offers an easy and very effective choice in rootkit removal, that is suitable even for beginners.
Sometimes the only symptoms you will get from rootkits is an increase in network traffic, or a decrease in performance, and maybe an unknown process running. So with today’s high bandwidth networks, and high performance computers it can be very hard to notice any signs. Prevention is always the best practice, but detection is just as important so make sure your AV has anti-rootkit capabilities, and make sure you have a good Firewall and HIPS combo. This and a combination of the tools I have mentioned are the best approach toward keeping your computer free of rootkits.