Can't access my SQL Express database

I'm trying to write a simple app, using VS 2010, to test accessing a SQL Server 2008 Express database.  I've tried using Windows Authentication, but that fails, all of the time.  Some time ago I created a dataset, but now I cannot use it, view the data, or anything at all with it.  Here's the message I get when I try to preview the data:
ASIDataSet.Providers.Fill,GetData() cound not be previewed.
Unable to open the physical file "C:\Databases\ASI.mdf". Operating system error 32: "32(The process cannot access the file because it is being used by another process.)" Unable to open the physical file "C:\Databases\ASI_log.ldf". Operating system error 32: "32(The process cannot access the file because it is being used by another process.)" Cannot open user default datbase. Login failed. Login failed for user 'MYDOMAIN\rod'.
I'm at a complete loss as to what the problem ...