I have bought a comp but its asking for a username and password which I don't have how do I remove it ***

Could be it wants you to programme it for a user name and password. That is the process and protection for your personal files. After you programme that into the computer no one can access your information when they turn it back on.

Where did you buy it? When does this username and password prompt? What Operating System?

This forum has specific rules about NOT offering help breaking the security of a computer or operating system.

If you bought a computer you cannot boot with normal workarounds and it's asking for passwords you cannot provide, you will need to reset the computer to factory settings. This would put the computer into the same state it was in the first time the computer was removed from the box, which should be fine for you.

This is usually done by pressing F10 multiple times during bootup, or Ctrl-F11... some keypress that activates the Recovery Partition hidden on most computers nowadays.

Provide your complete make and model of computer and we might be able to Google up some steps for you if you can't find them.

I bought it from last user

Then the last user can provide the login password.

Else search on the web to find out how to do a "factory restore" on that particular model of computer, resetting it back to the day it came out of the box will give you full access to clean, uncluttered computer.