Missing 49GB Hard Drive Space please help!

Ok I recently installed a new Motherboard and PSU in my Emachines W2888 that was previously dead (would not power up). I successfully reinstalled WinXP and everything was fine until a few days ago when the computer started acting up like it was on Win98. It became very stagnant, very slow. I checked the hard drive space and noticed that out of 80GB the system was only reading 31GB (rounded up). So the million dollar question is, where is the other 49GB? It says total used space 15GB, total free 16GB, total capacity 31GB.