Music lags when in other windows

Ok, this doesn't make any sense. I can have 100% cpu taken up by another program, but as long as I'm in the iTunes window there is no music lag. However, if I go to another window and 50% of the cpu is taken up, the music will start lagging. It's lagging a little bit right now but I have about 80% of my CPU free.
Another thing that doesn't make sense is that music that has been ripped from a CD or downloaded from iTunes tends to lag more, but music that has been downloaded (IE a band has a free EP for download, not necessarily illegal but I did download a for sale EP which I already own and deleted it later to test) doesn't.

So... what the living heck?

Are you using windows...? Or Mac OSx?

I had this on my old computer. I think it has to do with a not-so-powerful processor. As for the ripped music lag, it may be a problem with your disk drive. What are your import settings?

XP SP3, about to get a new computer which will have Windows 7.

Don't know what you mean by "Import Settings."

Yeah, it's your computer. I have no issues on my mom's Win 7 HP Dv7.

What laptop are you planning on getting?

Nevermind I don't think it's that. Your new computer shouldn't have the problem.