There are a few things that can cause this error. The first two are by far the most common:
1) Check in CMS under Common -> General Settings. Make sure the Windows User is set to a domain admin account, in the format DOMAIN\Administrator. The domain has to be in there. You may want to put the password back in while you're here. Sometimes that can get changed.
2) Make sure the Vexira account has the 'log on as service' right. To set this on the domain controller, go into Active Directory Users and Computers. Right-click on the domain and go to 'properties'. Under the Group policy tab, edit the default domain policy. Go to Computer Configuration -> Windows Settings -> Security Settings -> Local Policies. You want to make sure 'Log on as a Service' has the CMS user listed, in the same syntax (DOMAIN\Administrator).
3) If the above is set, make sure the client has inhereted the policy. You can check by going to Start -> Run -> 'gpedit.msc' on the client, and browing to the policy the same way.
4) If installing to Windows 2000, make sure SP4 is installed.
5) Make sure the machine is pointing to the right DNS
6) Log in as the user CMS is using and make sure you can browse to the C$ share on the client from the CMS machine.