BleepingComputer.com: Pc wont boot

Hi, i wonder if any one can help with this problem i have. I have an old PC that wont boot from hard drive. I get no error messages regarding this when the bios has done its checks, just a prompt to boot from CD. Having said that it does stay on the verifying data pool dialogue for a few minutes. However i have a UBCD 4 WIN Cd and if i select boot from hard drive instead of selecting the contents the pc boots no problem. I have checked the bios battery and cables to hard drive and all appears OK. System is keeping time etc. I have altered the bios to boot from hard drive first, previously it statred on floppy then cd then HDD. If i leave the cd in tray it prompts to boot from Cd and does so quite quicky, then i press boot from hard drive and everthing appears normal. Very strange.