Rootkits are a particularly secretive form of malware that go to extreme lengths to hide themselves from view. So while just about every antivirus tool includes some form of rootkit detector, these won't always be successful. To be sure you're free of infection you really need a specialist rootkit detector, and Tizer Rootkit Razor is one of the best, at least for 32-bit Windows.
The program is packed with features. It can scan running processes, your Registry, kernel files, services, ports, threads, the files or folders you specify, and more. Rootkit Razor also runs on all versions of Windows from XP through to 7, and (unlike much of the free competition) it's still being regularly updated and enhanced.
Most of the scans are very quick, too, most taking no more than a couple of minutes. The reports are straightforward, with any hidden objects being highlighted in red. And if you're sure there's a threat then in many cases you can take action immediately, for example closing a highlighted service in a couple of clicks.
You do need to be careful how you interpret what Rootkit Razor is telling you, though. If it locates a hidden Windows service, say, then this doesn't necessarily mean it's malicious. So don't panic just because you spot a few red lines, and don't start deleting any files it's specified: that could cause more problems than an actual rootkit.
In addition, keep in mind that Rootkit Razor offers on-demand scanning only. You don't get real-time protection, and the program isn't a substitute for a full antivirus program.