Has the little hand appeared on the icon of the shared drive?
It is not recommended that the system drive is shared but if you want to share it, go to My Computer > Right-click the drive to be shared > Sharing and Security > Click I understand the risks > Put a tick in Share this folder (drive) on the Network > Change the name (if you wish) > Tick Allow network users to change my files or leave it blank to give read access only > Apply, OK, etc. to finish.
If it doesn’t work you may have to turn off Simple File Sharing in Folder Options > View Tab > Advanced Settings at the end.
This probably doesn’t apply as you’re sharing with Vista at the moment but you may need to install the Link Layer Topology Discovery (LLTD) responder on the XP machine so that the computer shows up in Vista’s network map, get it here click here