I can think of two types of apps that could answer your question.
Portable versions of an antivirus. These are only used to scan once the OS has booted. I'm going to call these trash since a rootkit can easliy evade this.
Bootable Antivirus on USB. Now we're talking. Remember that no antivirus today is 100% effective and again we are talking SCAN ONLY as to be effective when the OS is running, we must install this to the OS.