Hi, please help me with the problem, my walls are showing up nail heads everywhere. I don't know why it doing that. It is a 15 years old home and I noticed the floor is starting to squeak too. How should I fix this problem? Do I pull out the old nails and screw in new ones. Should I hire a house inspector to check on the foundation? Thanks!:confused:

The nails are most likely showing up because they were not embeded into the drywall far enough, and they probably were not sufficiently covered over by drywall mudd.
One way to fix this is to use a nail set and set them farther into the drywall and then mudd over them twice, sanding in between coats of mudd and after the last coat of mudd.

A better way would be to pull the nail, and use drywall screws to replace rather than nails. After the drywall screw is in and slightly below the level of the drywall, you need to mudd over the whole.

Nail pops have little to do with not being set properly. The nail appears to pop because the lumber in the wall has moved due to moisture entering or leaving. The studs bow a bit and push the nails out of the drywall. This normally happens in the first 18 months so I'm not sure why a 15 year old house has started to pop. If you have large cracks in the drywall then you may have a foundation issue but pops can be repaired, the screw idea is the better option but don't bother pulling the old nail, just drive it home. I sthere any reason the lumber might be becoming moist or drier than in previous years?

Are the nail heads coming out of the wall or are they recessed? You've only said they are showing up. There are different reasons that can cause both of these. If they are recessed then there could be a problem with the sheetrock pulling away from the wall do to moisture soaked sheetrock or the drywall installers could have set the nails to deep (breaking the paper) whitch pretty much makes the nail useless. Drywall without paper is like a burrito without the tortilla. I would check for moisture first as stated above because it could cause a number of problems including yours. If the nails are popping out then follow ballengerb1s suggestions. Either way if your talking about a large area where this is happening I would highly suggest hiring an experienced drywaller to do the work it can be a lot more work than it looks.