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If your computer won't boot or is so damaged by malware that you can't trust it then you need something more powerful. This page covers useful stand-alone tools that can be booted from a CD or USB memory stick, leaving your broken operating system as a static image on your hard disk which you can examine with impunity.

Summary

A sick computer may not be able to boot, and even if it does, any malware that gets itself running before your software tools run can modify their behaviour to hide itself or give confusing results. Tools which boot independently of the installed operating system can treat it purely as static data and so give a completely objective view.

All the tools and utilities mentioned below should be easily found with your favourite search engine. As always, download from the official site, not a software download site, as such sites often install unwanted or undesirable software on the side.

Safety

Bootable utilities can cause data loss if misused, or if you're seriously out of luck. If possible, always ensure that essential data is backed up before starting work, and ensure the owner (if that's not you) understands and is content to run with the risk.

Boot managers

Plop is boot manager which can be useful if you can't get the BIOS to boot from USB and many other situations.

Hardware Diagnostics and Utilities

The following can be combined onto a single DOS-based memory stick or CD:

Spinrite (licence required) - this is a powerful hard disk and SSD data recovery utility.
MHDD - a dangerously powerful utility allowing low level diagnostic access to a hard disk.
Gold Memory - memory (RAM) tester.
memtst - another memory tester.
hdderase - a secure hard disk erase utility.

(Some of these are described under Advanced hard disk tools.)

Other utilities:

DBAN - Disk Boot And Nuke - bootable hard disk secure erase (see Disk Wipe Utilities)
Clonezilla - useful for backup and cloning hard disks.
Light weight Linux, e.g. Puppy Linux or Linux Mint Xfce edition. Useful to have gparted installed for examining hard disk partitioning.

(These need to be installed each on its own installation media, or they can be combined onte a single USB memory stick with Yumi.)

Bootable Rescue/Recovery Tools

(More details on these can be found in http://windowssecrets.com/top-story/emergency-repair-disks-for-windows-part-1/ - Windows Secrets subscription may be required.)

Hiren's BootCD - contains numerous utilities.
Trinity Rescue Kit - text-based, designed mainly for Windows but also including a Linux command line.
Ultimate BootCD - text-based, with over 100 repair/restore/diagnostic tools.
SystemRescueCD - Basic repair/recovery tools in a hybrid tex/graphical Linux environment.
Ubuntu Rescue Remix - almost entirely Linux command line based, offering a range of open source data recovery, repair and forensic tools.
Parted Magic - the collection of Linux programmes

Antivirus

Sophos Bootable Anti-virus
Kaspersky Rescue Disk - linux GUI based - periodically boot it to update signatures. Also useful for examining unbootable windows system.