Windows 7 – system error 5

My wife has just had a new computer for Christmas (with an upgrade from VISTA to Windows 7), and has joined the home network. We are using a mix of WindowsXP and Ubuntu boxes linked via a switch. We are all in the same workgroup. (No domain).

Internet access, DHCP, and DNS server is an SME server that thinks it is domain controller (although we are not using a domain).

I need to run a script to back up my wife's machine (venus). In the past the script creates a share on a machine with lots of space (leda), and then executes the line.

PSEXEC  \\venus -u admin -p adminpassword -c -f d:\Progs\snapshot.exe C: \\leda\Venus\C-drive.SNA

With the wife's old XP machine, this would run the sysinternals utility, copy shapshot,exe to her machine and run it, which would then back up her C: drive to the share on leda.

I cannot get this to work with Windows 7, nor can I link through to the C$ share on her machine. This gives me a permissions error (system error 5). The admin account is a full admin account. And yes – I do know the password.

The ordinary shares on her machine work fine!

I guess I'm missing something that Microsoft have built into Windows 7 – but what? The machine is running Windows 7 business, with windows firewall, AVG anti virus, and all the crap-ware you get with a new PC removed.

Added information

I've set that entry up (on venus) noting that System has a capital S, and rebooted, but I still cannot connect to c$. This is what happens.
C:\Documents and Settings\Ian>net use \venus\c$
The password is invalid for \venus\c$.

Enter the user name for 'venus': admin
Enter the password for venus:
System error 5 has occurred.

Access is denied.

C:\Documents and Settings\Ian>

There is a rather long pause after entering the password, before the System error 5 is returned.

Thanks