hi my western digital HDD 200 Gig is behaving strangely..

when i boot my hdd from my ubuntu OS it shows up and works fine. but when i boot my system in windows it shows the HDD as a 12 Gig HDD.
interestingly when i first boot the HDD in Linux (ubuntu) and then into windows (restart and not shut down and turn on) it shows the HDD as a 200Gig drive. but if i shut down my PC and then boot into windows it shows the drive as a 12 Gig drive. (even the bios reflects the above size)
help..
what could be the issue? i dont have any data on the drive so i can format it at will to resolve the problem.