bootkit cannot be detected easily because all its components reside outside of the file systems structure.
Some types of rootkits hide and lay hidden deep in the operating system. When the MBR has been compromised by a bootkit it can sometimes show up as a legitimate copy of the MBR.
A system infected with a bootkit can be cured with the TDSSKiller utility.
Cidox infects the load area code of the boot partition on the hard drive.