|Is this a desktop computer or a laptop computer?|
What brand name model is it, or of it's a generic system what model is the mboard?
"....we got out computer fixed."
Who was it who fixed it?
Someone who knows what they're doing would have made sure the correct sound drivers were installed and that speakers work.
Someone who doesn't know what they're doing may have installed the wrong sound drivers, or installed them the wrong way.
"....since we've gotten our computer fixed, I can't find "My Computer"."
There is no My Computer in Vista.
If you see My Computer listed at all, it's only because you have Hide Protected Operating System Files disabled in your Folder Options - View settings, it's only there for backward compatibilty, and it can't be accessed.
There's Computer - it's in the grey menu on the right when you click on the icon (Vista's Start) lower left.
There may also be an icon for that on your desktop screen.
If not, if you want an icon for it on your desktop screen, RIGHT click on Computer in the grey menu, choose Show on Desktop to make a checkmark appear beside it.
When you have a single pair of speakers, the speakers plug must be plugged into the green 1/8" port, and they probably must also be amplified - powered by an AC to DC adapter and switched on.