What model MBP?
What operating system is running?
You should realize that those programs legally belong to the company unless they gave you the original installer disks.
But one step at a time. Did they give you the original OS installer disks that came with that MBP?
Nothing to be scared about. A four year old can handle the Mac OS.
You need to create a new admin account in System Preferences, log out of any other accounts and log into that account, and go back to the Accounts (10.7 or below) or Users and Groups (in 10.8) panel and delete all the old users. If there's a chance you will need any of their files in the future, choose Save in Disk Image.
How can I do that safely without losing installed programs?
Unfortuntly most programs are tied to the owner, unless they were free or open source which isn't a problem getting new copies.
The only true way to erase the original owner off the machine is by a erase of the Macintosh HD partition and a fresh install of OS X, apps etc from new sources.
You can get by with making a new account on the machine, however for updates on certain software or if the police look at your machine it's still tied to the original owner and might be considered stolen until proven otherwise.