Test your PSU or replace it if your power supply units fan is not working your PSU is faulty

One bad lead can cause a computer to continue on a cycle or to shutdown or fail to detect/ boot up a computer hard drive

Test all leads that attach to your hard drive including electrical extensions,IDE,SATA

the leads from your ((motherboard to your hard drive)) make sure they have a secure connection and are not faulty or just replace them there probably old and faulty
make sure all leads that are attached to your drives dvd\cd 3 1/2 inch floppy have secure connections and are not faulty even the electrical extensions or just replace them they are probably old and faulty a computer needs its connections to have an end so any faulty leads will end up with a computer error

click start run type cmd then type chkdsk/r/f which will scan for and attempt to repair any bad sectors and automatically fix any corrupt files on your hard drive then defragment your hard drive or
control panel administrive tools,computer management ,disk management right click on your drive select properties click tools you should see click check now click start two boxes automatically fix files and scan for and attempt recovery of bad sectors place then a tick in both boxes then select check now,start should schedule this task when restart
then defragment your hard drive

hope this helps