Vista Partitioning Help

I have a 300 GB hard drive and windows vista. A few weeks ago i partitioned 40gb of it using the disk manager in windows vista to try out some beta software i had and when i was done i unpartitioned it using the same way back to the 300gb. i want to repartition my hard drive again now using the same way and it doesnt work. i go to computer management, right click my C drive and click shrink volume. i am able to select a volume that i want to shrink but then nothing happens after that. it just stays in a loading state without going to the next step. i've tried restartign my computer and the same problem happens. does anyone know how to fix this so i can partition my hard drive again?