how to install windows xp on used computer

Way is to buy your own copy of XP. It should have been eraced before it was given to you.
XP is per computer not user.

If this is a standard desktop, it is very possible that the license for XP is limited to the original purchaser only.

I myself would be concerned about security at work. It is no longer acceptable to release a computer without first erasing the hard drive.

Many places actually destroy hard drives from old computers just to assure that there is no way to recover any sensitive data.

There are a number of Windows password cracks that you can download and use to get into this system.

I myself would buy a copy of XP, boot from the CD and choose to format the hard drive.

Chas