There seem to be some difficulties with sculpting in Blender 2.48, try reverting to version 2.47 and remember "do not use the subsurf modifier in conjunction with sculptmode", use multires to raise the density of your mesh for detail sculpting. 

I f your issue bears on OpenGL versions you may even need to drop back to 2.46 in the event you can't upgrade your drivers to support the newer OpenGL version.

Even when sculpt works I've not been overly enthusiastic about it. I mean it's okay for gross work but it's just not on par with other available sculpting apps. The only advantage I can see is that it is "on hand" rather than having to change apps to sculpt.