i have a pc that is running a unlegit windows 7 os and am wondering would a upgrade pack work or will i have to buy the full legitimate
package

I doubt they'll let you upgrade without legit purchasing of 7.

..by... upgrade pack, I am assuming you mean like service pack 1 or whatever?... beeecause I have the most recent service pack. totally not legit here.

think it might be the service pack thingy i don't know any thing about them, the basic thing is that my dad bought a "costom" built pc with windows 7 ultimate 64 bit and a year ago it said it was not real and then this happened

I can confirm that you don't have to have an activated copy of windows to run Windows 7 as an upgrade from any other version. All the upgrade requires is for a version of windows to be installed before you run the setup disk. Once it is able to compact that installation into a 'windows old' folder on your drive, you're golden.

For example. Even for a new build PC with no data on it - Run the windows 7 installer to install the OS. Now if you try to activate it, it will tell you that the product key isnt valid for a fresh install - it is an upgrade copy after all. So all you do is run the install disk again, and install over the top of it so it creates the 'windows old folder' . Now Windows thinks it's an upgrade and now your product key WILL work!