Does the drive appear ok in My Computer? You could try deleting the drive, restarting windows when windows will reinstall it by itself and see what that does. If that doesn't help and if you are sure the disks are ok then I would say the liklihood is the drive is starting to fail and you might need a replacement. Fortunatly DVD ROM drives, and I assume this is what you have are cheap now, as an example click here if you have a cd rom cleaning disk you could try that in case its just a bit of dirt on the lens. Or if you want to replace it with a DVD writer click here though if you go this way then you are best picking a writer that includes burning software if you no longer have it.
Wait and see if anyone else comes up with something, perhaps its a simple solution I have over looked.