Unless there is a security or 'fiddler' issue, I would use administrator rights for each comp as the norm - it saves a huge amount of hastle. I don't even bother with a password to open the default profile.
If you are sure that you have administrator rights when you tried to share drives, I'm at a bit of a loss.
The only thing I can suggest is that you try the following:
Double left click on 'My Computer' on the desktop.
Right click on the drive to be shared.
Left click on 'Sharing and Security'.
Left click on the 'Sharing' tab.
Under 'Network Sharing and Security' there are 2 tick-boxes. Untick these and then click on 'Apply'.
Now re-tick those 2 boxes and click 'Apply' then 'ok'.
With a bit of luck the 'sharing hand' should appear under the disk symbol.
To check, open Windows Explorer, expand 'My Network Places' and the disks shared will be listed by name; it should include the disk you have just re-done.
Try that and let us know how you got on...
FJJP