Help please. booting problem

so i have windows 7 installed on my computer and basically when i went to turn it on, it froze at the part where the logo comes up and it says Starting Windows underneath. this isn't the first time that this has happened. earlier, it froze at the same spot, but finished loading a few minutes later. it's been over 5 mins now, but nothing has happened. what could be the problem? how can i fix this?
im not sure if i should even turn it off now. and if i can, where should i turn it off from (power button, pull the plug etc.)?
now it's gone to a black screen that says No boot device availableSATA 0: InstalledSATA 1: Installed SATA 2: NoneSATA 3: None

a little info of the computer. i don't know much about computers, but this one is a dell inspiron 531. was originally running on vista. problem had happened when vista was installed. new power source put in after the first time it messed up. messed up again. hard drive was cleaned, same thing happens a week later. new hard drive installed (i'm assuming this is how i got windows 7). works for about a month and it messes up again.

i don't know if anything else has been changed

Edit: can someone move this thread. just realized it's in the wrong section
 
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