Bitdefender Rootkit Remover is a quick and easy tool for dealing with known rootkits.
The company say it can currently remove Mebroot, all TDL families (TDL/SST/Pihar), Mayachok, Mybios, Plite, XPaj, Whistler, Alipop, Cpd, Fengd, Fips, Guntior, MBR Locker, Mebratix, Niwa, Ponreb, Ramnit, Stoned, Yoddos, Yurn, Zegost and Necurs.
There's no need to install anything, and so no worries about conflicts with other security tools. Just download the program, click "Start Scan", and it should take care of everything else.
What you will notice, if you're not infected by anything, is that scan times can be extremely fast - perhaps just a second or two. And that's because Rootkit Remover works with known threats only, so there's no need to examine every file on your system - it already knows exactly where to look.
Of course this also means that Rootkit Remover is no substitute for a full antivirus engine, something which might be able to prevent your system from being infected in the first place. But if you think you may have a rootkit and your current security package can't help, then it may be a good idea to give Bitdefender Rootkit Remover a try.