Mostly all laptops can be unlocked by master password which unique for every laptop
Mostly all desktops may be unlocked by removing coin battery or by special jumper.
Laptops have EEPROM which cannot be cleared by removing battery.
Many customers forgot their passwords, sometimes it laptops from ex-employee or purchased on garage sale.
Many computer models can be unlocked by master password which depend from password, serial number or even random generated after special procedure.
Master passwords can be generated by special tools, mostly all are not worldwide available, very few persons have it.
More often case is computer bios by Phoenix, which use 5-digit code after 3 unsuccessful tries to generate master password. Bios from Phoenix used by practically all brands with some modifications in the code calculation. For HP Omnibook and HP Pavilion to get 5-digit code need to press F2 F12 at password prompt.
For example usual hash code:  password: 1134334
HP models hash code:  password: 1221196
[DELL service tag]
Dell use service tag to generate master password. Usually it ended with —595B.
Some models used modified algorithm for —A95B service tag which sometimes differ from —595B
Obsolete models may use —D35B tag for that exist freeware utility
Inspiron 700m and 710m use only service tag without suffix like —595B
Very old models have special master passwords but such models are junk now.
Master password generate separately for BIOS and HDD, because for HDD used its 11-letters serial number.
For old Dell laptops exist so called Paperclip method, it works but dangerous, many laptops are bricked now
service tag: 1234567—595B password: 46rg65ky
service tag: 1111111-A95B password: 2qhuvn1b
service tag: 1234567-D35B password: 5tc8q9re
HDD serial number: 123456789AB-595B password: 76sf9hsl
HDD serial number: 123456789AB-A95B password: 5m8oalqm
Inspiron 700m:  password: 37897101
Sony PCG series can be unlocked by last 7 digit from serial number
Sony VGN-A series after 3 tries it generate random xxxx — 4-hex code
Sony VGN other series after 3 wrong tries it generate random 5×4 code, do not turn off laptop until get master password
For example: Sony VAIO VGN-SZ22 [268r-dkxj-38j7—9j2k] password: 33637436
This brand use many different algorithms to calculate master password
Main code: xxxx-xxxx-xxxx-xxxx-xxxx 5×4 digit, to get it enter 3 passwords one by one:
Do not enter again passwords until you get master password
Other variants is 5-digit code, 8-digit code, 5×4-hex code, need to know exact model as many models has different calculations.
Some models have PIN-lock panelm which can be unlocked by erasing concerned EEPROM
For example: LifeBook Q2010: 1111—2222—3333—4444—5555 password: 8i4wjlon
For example: Amilo Pro V3205: 12345678 password: 2171081
Asus laptops use current date which show at system start up to generate masterpassword.
For example: 2007—12—31 password: LAOLDOA0
Mostly used Phoenix 5-digit code to get master password, various algorithms
v1  password: 1134334
v2  password: B5P1HA7
v3  password: 1123155
v4  password: 03023278
HP use classic Phoenix 5-digit code and HP modified 5-digit code
There is also HP-Compaq models, which can be unlocked by manipulations with battery and charger
For example: System Hash Code  password: 1221196
Samsung use 16-hex code which appeared after 3 unsuccessful tries to restore forgotten password
For example: System Disabled [153E0FC8C7B03144] password: SAMSUNG
Old laptops can be unlocked by floppy key disk, LPT key, USB key.
Current models use Challenge method: at password prompt press keys one by one: CTRL, TAB, CTRL, ENTER. Computer will respond with PC Serial number and Challenge code. You have to respond with Response code
Some Toshiba use Phoenix 5-digit code, but it appear for very short time, you have to use camera to catch the code.
Challenge method have at least 3 modifications.
1) for laptops like Qosmio F10, all algorithm located inside BIOS except one calculation on the other side
2) for laptops like Tecra M4, core of algorithm hidden inside of EC/KBC chip with encrypted firmware
3) the same laptops but with corrupted eeprom contents which connected to EC/KBC