Rootkits allow a malefactor to take control of another's system. There are free programs to help detect rootkits on a Windows system. Two such programs are BlackLight from F-Secure and RootkitRevealer from Sysinternals, which has been acquired by Microsoft.
To use BlackLight, simply download it and run the downloaded file. There is no installation process. When the scan is completed, BlackLight will report whether it found any hidden processes, files, or folders that may be part of a rootkit.
Analysis of the results of a RootkitRevealer scan requires more technical competence as some of the entries you see listed in its report may be normal for Windows systems and not necessarily a sign that the system has been "rootkitted".