partition main drive

i'm an absolute newbie to linux. i installed ubuntu 8.04 last saturday. until then i had vista on drive c and a separate partition for all my files. i formatted c. i was not aware that ntfs format disks were not compatible with ubuntu. as could be expected, i ran into a problem when i tried to install ubuntu onto that ntfs partition. out of a mix of desperation and excitement, i chose the option to use the entire hdd (120gb). i had a feeling that i did something wrong, and i was right. all my files on d disappeared. but that's not my problem. i have backups of the files, anyway. now i want to resize c. i downloaded and installed gparted, but no go. the options to resize partition are grayed out, and it says unmounting c was not possible (for obvious reasons, i think, that it is holds the os). i also tried to use partition magic, but it warned me about some state of the hdd and correcting the "state" may cause data corruption, so, i held back on doing anything. is there a way i could still resize my hdd? please help.