can any one help me, im trying to backup a database located on localhost\SQLEXPRESS but i keep getting the following error

TITLE: Microsoft SQL Server Management Studio

Backup failed for Server 'localhost\SqlExpress'. (Microsoft.SqlServer.SmoExtended)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.0.2531.0+((Katmai_PCU_Main).090329-1045+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Backup+Server&LinkId=20476

ADDITIONAL INFORMATION:

System.Data.SqlClient.SqlError: Cannot open backup device 'C:\backup.bak'. Operating system error 5(failed to retrieve text for this error. Reason: 15105). (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.0.2531.0+((Katmai_PCU_Main).090329-1045+)&LinkId=20476

can any one explain what im doin wrong here ??

thanks