Umm, is your disc(s) scratched? If so, get a Disk Doctor (Game Dr., Skip Dr., DVD Dr., whichever's cheapest) and repair it. Then try re-installing it.
If your disc isn't scratched, you might have dust in your CD or DVD drive. Try buying a can of compressed air (Wal-Mart, Lowe's, Home Depot, Staples, etc) and using that to blow the dust out of it. (DON'T SPRAY UPSIDE DOWN!)
PS
www.walmart.com and search for
skip doctor or
4062502 That one is around $30 including shipping. I'm sure there are others that work, but I know from personal experience about the Disk Doctor (I have two!).
PPS If you hold your disc up to a light, with the label side facing
away from you, and you can see hole(s) scratched
through the label, then you're toast. No can fixee.
See if any of these ideas work. If not, let us know.