Rootkits are particularly nasty pieces of malware that can not only compromise your computer, but will hide their tracks, so detection and removal becomes particularly difficult. The Win32:Popureb.E variant is a good case in point, after admission by Microsoft that not only will you need to do a complete OS reinstall to get rid of this nasty bootkit, but you will need to fix your master boot record before doing so; something that may be beyond the capabilities of novice users.