I have someone who downloads programs i dont want on my pc. How can I set a pword to approve/deny downloads?

This has happend numerous times and this person will not accept no for an answer-i delete program then it is magicly reappears. I dont mind this person using my pc so i grant them access as needed..I have a lenovo with vista-. I need to be able to set pwords for my personal downloads only- can this be done? Thanks