Without knowing exactly what operating system you are using then the chances of answering that question zero. It would be handy to know what computer you have as well.

 

I am tring with win xp sp1. And my machine config
Motherboard: ASUS M2NPV-VM
Processor: AMD X2 4200+
Harsdisk: SEGATE 250 GB SATA
RAM: Trascend 1 GB
OS: WINXP Pro. SP1

 

Now we have some information your problem is obvious! Windows XP sp1 cannot read hard drives bigger than 128 gig. What you need to do is go to the Microsoft website and update your version of Windows to service Pack 2.
Once you have done this you should get full capacity of the hard drive.

 

Thank a lot for your help, I will try to get SP2 now

 

Keep in mind that even after you have updated to SP2 your harddrive still wont show up as 250GB. I will most likely be somewhere around 238GB and this isn't the fault of Windows, but because of the fact that harddrive manufacturers use MB, but everyone else uses MiB. What Windows calls MB is actually MiB.

1,000,000 bytes = 1Megabyte
1,048,576 bytes = 1Mebibyte

If you have already formatted the drive with one 125GB partition you will after the update either need to remove that and create a new partition that spans the entire drive or create a new partition that uses the left over space that the other partition doesn't.