:)
Which are the measures to be taken to make the computer boot faster ? Does formatting of hard disk has any negative effect other than losing of data ? What is the benefit of formatting the hard disk ?

Post #3 here gives maintenance details: https://www.askmehelpdesk.com/computers-beginners/frequently-ask-questions-about-computers-233870.html

As for your other questions.
An occasional rebuild, i.e. complete format and start over, can improve the speed and responsive of your machine.
Preparation is key in this case.
Make sure you have all of your data backed up and all installation media (CD's) readily to hand.

Turning off unneeded services helps.
If you monitor what your computer is booting/running well, you'll notice any slowdowns quicker. Search for help on how to show what's booting, and if it has any errors, which will loop or slow down boot.

As mentioned above

Hacking Windows XP: Speed Up Your Boot

Disabling Unneeded Services
Hacking Windows XP: Speed Up Your Boot - Disabling Unneeded Services - OS, Software & Networking by ExtremeTech (http://www.extremetech.com/article2/0,1697,1786006,00.asp)