I just bought a new Western Digital 250gb OEM hard drive, and when I installed it said it only had a capacity of 127gb. Is there any way I can find out where the rest of my storage space is and if so how can I get it back. If I can get it back is there anyway to do that without losing everything i've already put on there; I have back-ups so I can if I have to. Any help me

It's a common problem when installing large hard drives with certain motherboards. I had the same problem with a 120GB a couple of years ago, it's just a case of informing the mobo that the drive is larger then it thinks it is.
As far as WinXP is concerned drives larger than 137GB do not exist.. but thsi can be remedied by installing support for 48-bit LBA (Logical Block Addressing).
Service Pack 1 is required for this fix, so i'll assume you have it as it's been out for a while.
A full explanation on how to enable 48-bit LBA can be found from the good people who messed this up in the first place

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-William. § (marvin_gohan)

you will need service pack 1 to solve this problem. i advise u to load service pack 2 also as it will be beneficial for your future use.