I just downloaded the Everybody can play guitar mod. When I tried extracting it to my mods folder in the Sims 3, I get this looong error.
C:\Users\Boorsman\AppData\Microsoft\Windows\Temporary Internet Files\Content.iE5\PY1G287L\MTS_bella3lek_94359_everybody_can_play_guitar(1)zip:Cannot create package
Access is denied.

I tried extracting it directly to C:\Program Files\Electronic Arts\Sims 3\Mods. That's when the error comes. I tried opening the zip and dragging the packages over to the mods folder, and they don't work in my game. Am I missing something? Am I doing it wrong?

Have you followed the wiki guide on how to set up your mods and packages folder in the "program files > the sims" folder? if you have, then just do it manually. Extract the files from the zip and put them somewhere, it doesn't matter where (desktop is probably most convenient). Then just drag them all into your "mods > packages" folder. They should work provided you've set it up as the wiki says to.

edit: also, check you have admin privileges on your account. That has been giving people problems too.

It definitely sounds like a Vista system not letting you put things where you'd like.
I just download stuff to my desktop and unzip there. Then I can grab the file and put it in the mods package just as haylz320 says. That's what works for me.