If you forgot your Windows password and you don't want to reset it, you have a new option: Ophcrack LiveCD, a Windows password cracker that claims to be able to crack 99.9% of alphanumeric passwords in seconds. Ophcrack is based on a time-memory trade-off using rainbow tables, some precalculated data stored in memory. A rainbow table is a lookup table that is constructed by placing a plaintext password entry in a chain of keys and ciphertexts, generated by a one-way hash (a function that is easy to calculate but hard to invert). The end result is a table that contains statistically high chance of revealing a password within a short period of time, generally less than a minute.
Minimum amount of RAM required: 256MB.
Recommended amount of RAM: 512MB.
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Rainbow Tables (Wikipedia)