During startup press F8, from the options choose safe mode ( in this only the absolutely essential processes are loaded, so defragmenter can function without interruption). Then run scan disk, as the defragmenter will not run if there are bad sectors on ur drive. After that defragment( your system will have the defragment utility- check in system tools). The defragmenter moves fragmented files back together into continguous chunks, thus making file access faster for the HDD. If you dont defragment, the system is bound to become slow.