I assume you have Windows Vista.

Try this:
http://support.microsoft.com/kb/927392

Let us know how that goes. 

Hi i have a dell dimension c521 my old hard drive wont turn on so i got a new hard drive but when i turn on the power i get this messige (no boot device detected insert system disk and press f1 to continue or f2 to setup menu) i have to press f12 on first screen to boot cds and the os but when i turn off the computer it wont let me run the os so i have to reinstall to start it up every time  so i ran some tests it said start up missing boot maniger    Please Help AS Soon As

 brad1000

After installing the new SATA hard drive, did you reinstall windows and load all the drivers?

Bev.

If I were a betting man I'd say that's the issue.

LOL.   :emotion-22:

Bev.

[quote user="brad1000"] Hi i have a dell dimension c521 my old hard drive wont turn on so i got a new hard drive but when i turn on the power i get this messige (no boot device detected insert system disk and press f1 to continue or f2 to setup menu) i have to press f12 on first screen to boot cds and the os but when i turn off the computer it wont let me run the os so i have to reinstall to start it up every time  so i ran some tests it said start up missing boot maniger    Please Help AS Soon As

 brad1000

After installing the new SATa hard drive, did you reinstall windows and load all the drivers?

Bev.

[/quote]

If I were a betting man I'd say that's the issue.

yes i installed all drivers after installing the os but it wonted load after turning the computer off :emotion-6:

I just tryed Bootrec.exe then tryed to install vista got error message when it was on expanding files code 0x80070017