1. A method, including: booting a computer system into a secure preboot environment maintained at least in part on a removable storage device coupled thereto, said secure preboot environment including instructions directing a processor to perform integrity checks against malware, said secure preboot environment preventing unauthorized access to said instructions; performing the integrity checks by executing the instructions on said processor, wherein the integrity checks include a system file integrity check for a set of system files on said computer system and an anti-virus scan on a list of files obtained from the system files that have passed the system file integrity check; and upon completing said integrity checks, rebooting said computer system into a portable operating environment maintained at least in part on the removable storage device, said portable operating environment including operating system software for controlling said computer system and for allowing execution of application program software maintained on the removable storage device.