Here's a great write-up with screenshots on how to resize and extend a Windows 2003 "C" drive on VMware vSphere.
http://thevlab.net/vmware/tips-and-tricks/extending-a-vsphere-win2k3-os-drive/
Link for the Dell Utility (called ExtPart) that he uses:
http://support.dell.com/support/downloads/download.aspx?c=us&cs=19&l=en&s=dhs&releaseid=R64398&formatcnt=2&fileid=83929
Hope it helps!