PC doesnt recongize my memory stick as a formatted memory stick...

My 2 gig memory stick for my psp is already formatted, and works perfectly on mylaptop, but not my PC. The PC recongizes it as a psp, and a memory stick, but it gives me a message saying that its "not formatted". i already formatted it on the PSP, and worked on the laptop..and when i try to format it on the comp, it says it cant..whats the problem? any help would be appreciated. (i run windows XP professional btw)