How do I get a program off of my computer?

How do I get a program off my computer if it blocks everything I do. is there any way I can download something to over power the program and let me do whatever I want...

I Must ask first Who put the program on the Computer and what is it called.
And I'm asking anyone else answering to respect this.

(added for clarity):

A program removed incorrectly can crash a system or make a ghost pop up
Ghost pop up - An entry in the System Registy that trys to start a program when the system is started Being able to freeze up the machine at times.
Or it may be spyware.
mischiev0us91 :
I also need to know the name of the program. . . . .

your right...but an easier way is to know what system he/she's running on first don't ya think?