1)first check in BIOS. if it is detecting 250GB there,then you need to fix LBA (logical block addressing) in your OS.
if bios is detecting 127 GB ,then you need to update your bios.
(if your operating system is using "36 bit LBA" then maximun harddisk size it can detect is 127.4 GB.to detect more than 127.4 GB ,the OS need "48 bit LBA")
2)download "EnableBigLba.exe" its size is 156 kb and its freeware. run it and it will make LBA 48 bit. reboot.
3)or to manually do it.
* regedit
* HKey_Local_Machine\system\current Control set\services\atapi\parameters
* find "EnableBigLBA" there and change it value to 1.(if it is not there,write it and make its value 1)
* exit Regedit and reboot.
(note: installing new SP2 patch can also fix this problem.)