new motherboard not working

hey,i am currently on a different computer atm and i just bought a new motherboard.
the info:athlon 2800 motherboard bundle
was just putting pieces from my old comp into the new motherboard(video card,hard drive,sound card.
first time it worked but hard drive was not connected properly,when i was trying to put windows back on to reinstall it for my new motherboard,it said:"hard drive not detected" so i pulled the black long slim plug out and put it in again.next time i booted my computer it just booted with showing the properties of the computer and the screen after it just said:"error loading system".

please help

new problem.now ive finally sorted my computer,all it DOES do is restart.
what i did when i got it up and running:updated drivers,installed oblivion,installed guild wars,got on guild wars(and started screaming YAY!! on all my friends whisp+chats) about 2 minutes later it restarts,tried it again and it does the same,so on and so on.

does the hard drive already have Windows installed on it? (seeing it was from your old comp after all)

if you can get into the BIOS, try setting your CD/DVD drive as the first drive to boot from, stick the Windows CD in there (or any bootable CD), and see whether it does anything.

save my 100 quid!