After intall fresh copy of windows xp,c:\ space reduce 48gb to 22 gb.how can solve it?

Sounds like you have two copies of windows on your hard drive.

^^ as above.
Make sure you dont have a dual install

That would be my guess as well.

Generally, re-installing Windoze requires “the nuclear option”; formatting the drive entirely. Trying to merely re-install may easily lead to a dual-install. I have done it myself a couple of times by hitting the wrong menu option at 2AM. I think my record it three installs at the same time :P

Delete the partition and reformat it before installing.